What redlining means and why you'd use it

Redlining is marking changes directly on a document so the original author can see what you changed and why. In a PDF, redlines appear as colored lines, boxes, or strikethrough text — usually red, though the color is just convention. You might redline a contract to flag a clause you want removed, mark up a manuscript to suggest rewording, or highlight numbers in a spreadsheet that don't match your records.

The key difference between redlining and just editing: redlines leave the original text visible and add your mark on top of it. The document owner can accept or reject each change one at a time, rather than getting a version where you've already deleted things. This matters most when multiple people are reviewing the same document, or when you need a record of what changed and who suggested it.

Most PDF editors let you add redlines, but the tools vary widely — some are built into free readers, others require paid software, and a few work only in your web browser. The method you choose depends on what software you already have, whether you need to send the marked-up file back to someone else, and how much control you want over what information the file contains.

Key Takeaways

  • Redlining in a PDF means adding visible marks (lines, boxes, strikethrough) over text without deleting the original, so changes can be reviewed and accepted or rejected individually.
  • Free PDF readers like Adobe Reader and Preview (on Mac) include basic redline tools, but paid editors like Adobe Acrobat Pro offer more control and better compatibility when files are shared.
  • Browser-based editors like Google Drive and Smallpdf let you redline without installing software, though they may store your document on their servers unless you use privacy-focused alternatives.
  • When you redline a PDF, metadata (your name, timestamp, device information) gets embedded in the file unless you strip it out, which matters if privacy is a concern.
  • The person receiving your redlined PDF needs compatible software to see your marks — not all readers display comments and strikethrough the same way.

Using the free tools already on your computer

If you use Windows, Adobe Reader (the free version) includes a "Draw" tool that lets you add lines and boxes to a PDF. Open the PDF, click the pencil icon in the toolbar, then select the line or box tool. Draw directly on the page where you want the mark. You can change the color and thickness, though the default red works fine for redlines. When you save the file, your marks stay in it.

The limitation: Adobe Reader's drawing tools are basic. You can't easily add a strikethrough to text, and you can't add a comment explaining why you made the mark. If you need to explain your changes, you'll have to add a separate text box with your note, which clutters the page.

On a Mac, Preview (built into every Mac) does more. Open the PDF, click the markup toolbar icon (it looks like a pen in a circle), then choose from highlighter, pen, or text box. Preview also has a strikethrough tool under the "Sketch" menu. You can add a comment box that pops up when someone hovers over it, which is closer to professional redlining. Save the file when you're done — your marks are permanent unless you delete them.

Both tools work fine for straightforward redlines you're sending to one person, but neither gives you a clean way to track who made which change or when, which matters if the document passes through multiple reviewers.

Using Adobe Acrobat Pro for full redline control

Adobe Acrobat Pro is the professional standard for redlining because it was built for this task. Open your PDF, click "Tools" in the top menu, then "Comment." You'll see a toolbar with strikethrough, highlight, underline, and text insertion tools. Click the tool you want, then click or drag on the text you want to mark. A comment box appears where you can type an explanation.

The advantage: Acrobat Pro tracks your name, the date and time of each mark, and lets you organize comments by type. When someone opens the marked-up file, they can accept or reject each change individually, and Acrobat will show them a summary of all changes. If multiple people redline the same document, Acrobat keeps each person's marks separate and color-coded.

The cost is the main barrier — Adobe Acrobat Pro is a paid subscription (roughly $20 per month or $240 per year, though prices vary by region and promotions). If you redline documents regularly as part of your job, the cost is usually worth it. If you do it once or twice a year, the free options below may be better.

One more thing: Acrobat Pro embeds metadata in the file (your name, email, device, timestamps). If you're sending the file to someone and privacy is a concern, you can strip this data before sending, but you have to do it manually.

Free online editors that don't require installation

Google Drive lets you upload a PDF and mark it up in your browser without installing anything. Upload the file to Google Drive, right-click it, and select "Open with" then "Google Docs." Google will convert the PDF to a document format. You can then use the comment tool (click "Insert" then "Comment") to add notes, or use the suggestion mode to mark changes. When you're done, read it as a PDF.

The catch: Google's conversion isn't perfect for complex PDFs with images, forms, or unusual layouts. The converted document may look different from the original. Also, Google stores your file on their servers by default, which matters if the document contains sensitive information. You can delete it after downloading, but it exists on Google's servers while you're working on it.

Smallpdf and ILovePDF are browser-based PDF editors that let you add text boxes, strikethrough, and highlighting without uploading to a personal account. Go to their website, upload your PDF, use their markup tools, then read the marked-up file. Your file is deleted from their servers after a few hours (check their privacy policy for exact timing). These tools are simpler than Google Docs but also less powerful — you can't easily add comments that explain your marks.

All browser-based tools have one advantage: they work on any device with a web browser, including phones and tablets. They have one disadvantage: if the website goes down or changes its terms, you lose access.

What happens to your personal information when you redline

When you add a comment or redline to a PDF in most editors, the software automatically embeds your name, the date, the time, and sometimes your email address or device information. This is called metadata. It's useful if you're collaborating with colleagues who need to know who suggested each change. It's a problem if you're sending the document to someone you don't want to know details about you.

In Adobe Acrobat Pro, you can remove metadata before saving. Go to "File" then "Properties," and you'll see fields for Author, Subject, and Keywords. Delete the information you don't want to share, then save. In Preview on Mac, metadata is minimal — it usually just includes the timestamp. In Google Docs, your Google account name is visible in comments, and you can't easily remove it without deleting the comment itself.

If you're redlining a document that will be shared widely or published, consider whether you want your name and timestamp visible. If not, check your editor's privacy settings or metadata removal options before you send the file.

Making sure the person who receives your redlines can see them

Not all PDF readers display redlines the same way. Someone using an old version of Adobe Reader might not see strikethrough marks you added in Preview. Comments might not show up at all in some mobile PDF apps. Before you send a redlined PDF to someone, ask what software they use to open PDFs, or send them a test file first.

The safest approach: use Adobe Acrobat Pro or Adobe Reader to create your redlines, since those are the most widely used and most compatible. If you use Preview on Mac or a browser-based editor, mention in your email that they should open the file in Adobe Reader or Acrobat to see all your marks clearly.

If the person receiving your file doesn't have compatible software, you have two options. First, you can export the redlined PDF as a flattened image (which locks the marks in place but makes them impossible to edit). Second, you can create a separate document listing all your changes in order — slower, but it works in any software.

Redlining forms and fillable PDFs

Some PDFs are forms with boxes you fill in. Redlining a form is different from redlining a text document because the form has a fixed structure. If you want to mark up a form field, use the same strikethrough or highlight tools you'd use on regular text — draw the mark over the field itself, not inside it.

If you're reviewing someone else's filled-in form and want to flag an error, add a comment box next to the field rather than trying to strikethrough the answer they gave. This keeps the original answer visible while making your concern clear. If you need them to change the answer, say so in the comment.

Forms created in Adobe can sometimes be edited directly (you can change the text in the field itself), but this isn't redlining — it's overwriting the original answer. Stick to marks and comments if you want the original to remain visible.

Frequently Asked Questions

Can I redline a PDF on my phone?

Yes, but it's awkward. Most phone PDF apps have basic markup tools — you can usually add text boxes and highlights. Adobe Reader and Preview both have mobile versions with redline features. Browser-based editors like Smallpdf work on phones too. The limitation is screen size and precision — marking up text with your finger is harder than with a mouse or trackpad.

What's the difference between redlining and highlighting?

Highlighting colors text but leaves it readable and in place. Redlining usually means strikethrough (text with a line through it), though it can also mean adding marks like boxes or underlines. Strikethrough shows that you want text removed; highlighting just draws attention to it. Use strikethrough when you want something deleted, highlighting when you want to flag something for review.

If I redline a PDF, can the original author undo my marks?

Yes, if they have the right software. In Adobe Acrobat Pro, the author can reject individual changes or accept them. In most other editors, your marks are permanent once saved — the author would have to manually delete them. If you want the author to be able to undo your changes easily, use Adobe Acrobat Pro or ask them what software they prefer.

Does redlining change the file size of the PDF?

Yes, slightly. Each comment and mark adds a small amount of data to the file. If you add many comments with long explanations, the file size can grow noticeably. This usually doesn't matter unless you're emailing the file and have a size limit, but it's worth knowing.

Can I password-protect a redlined PDF so only certain people can see my marks?

Most PDF editors let you add a password to open the file, but that protects the whole file, not individual marks. Adobe Acrobat Pro has more granular controls — you can restrict editing or printing — but you can't hide specific comments from specific people. If you need that level of control, you're better off sending different versions to different reviewers.