The simplest way to merge PDFs depends on what you already have

You can merge two PDF documents using free tools that live in your browser, tools built into your operating system, or desktop software — and which one works best depends on whether you need this done once or regularly, and whether the PDFs contain sensitive information.

If you have just two or three PDFs to combine and they are not confidential, a browser-based tool like ILovePDF, Smallpdf, or PDF Merge takes about two minutes. If you work with PDFs constantly or handle documents with personal information, a desktop tool like Adobe Acrobat Reader (free version) or an open-source program like PDFtk gives you more control and keeps your files on your computer instead of uploading them to a server.

Key Takeaways

  • Browser-based PDF mergers are fastest for occasional use but require uploading your files to someone else's server.
  • Desktop tools like Adobe Acrobat Reader (free) or PDFtk keep your files private and work offline, but take longer to learn.
  • Mac and Windows both have built-in PDF tools that can combine documents without installing anything new.
  • The order matters — most tools let you drag and drop pages to arrange them before merging.
  • Merged PDFs are usually smaller than the originals because the tool removes duplicate data, but file size varies by content.

Using a browser tool when you need speed

Browser-based PDF mergers work on any device with an internet connection and require no installation. You upload your PDFs, arrange them in the order you want, and read the merged file in seconds. ILovePDF, Smallpdf, and PDF Merge are the most common options, and they all work the same way: drag your files into the box, reorder if needed, and click merge.

The trade-off is privacy. Your PDFs travel to the company's server, get combined there, and then you read the result. Most of these services say they delete files after a few hours, but you are trusting their word and their security practices. If your PDFs contain bank account numbers, medical information, or anything you would not want on someone else's computer, use a desktop tool instead.

These tools are free, though some offer paid versions that let you batch-merge many files at once or remove ads. For merging one or two PDFs occasionally, the free version is enough.

Using your computer's built-in tools

On Mac: Open Preview (the app that opens when you double-click a PDF). Go to File > Open and select the first PDF. Then go to File > Open Again and select the second PDF — it will open in a new tab. Click the thumbnail panel on the left side of the first PDF, then drag the thumbnail of the second PDF's pages into that panel in the order you want them. Go to File > Export as PDF and save the merged document.

On Windows: Windows does not have a built-in PDF merger in the same way Mac does. Your options are to use the free version of Adobe Acrobat Reader (see the next section) or to use a browser tool. Some versions of Windows come with Microsoft Print to PDF, which can combine documents if you print them to a single PDF file, but this is slower and less reliable than other methods.

Built-in tools are free and keep everything on your computer, but they are slower than browser tools and less flexible if you need to do this regularly.

Using Adobe Acrobat Reader (free version)

Adobe Acrobat Reader is free to read and install on Windows, Mac, or Linux. The free version includes a tool called Combine Files that lets you merge PDFs without uploading them anywhere. Open Acrobat Reader, go to Tools > Combine Files, click Add Files, select your PDFs in the order you want them, and click Combine. The merged PDF opens in the editor, and you can save it.

The free version of Acrobat Reader has limits: you can combine files, but you cannot edit the text inside them or add passwords. If you need those features, Adobe sells Acrobat Pro (a paid subscription), but for merging alone, the free version is sufficient.

Acrobat Reader is reliable and widely used, so if you run into problems, you can usually find answers online. It also works offline, so you do not need an internet connection.

Using PDFtk for people who work with PDFs often

PDFtk is free, open-source software that runs on Windows, Mac, or Linux. It is more powerful than the other options but also more technical — you use it from the command line (a text interface) rather than clicking buttons. If you regularly merge dozens of PDFs or need to automate the process, PDFtk can do that. If you just need to combine two documents once a month, it is overkill.

To use PDFtk, you read and install it, then open your computer's terminal or command prompt and type a command like pdftk file1.pdf file2.pdf cat output merged.pdf. The files merge in the order you list them. You can also use a graphical interface called PDFtk GUI if you do not want to type commands.

PDFtk is free and keeps your files private, but it has a steeper learning curve than the other options. Use it if you are comfortable with command-line tools or if you need to merge PDFs as part of a larger workflow.

What happens to file size when you merge

A merged PDF is usually smaller than the sum of the original files because the merging tool removes duplicate data — fonts, images, and formatting that appear in both documents get stored only once in the merged file. A 5 MB PDF plus a 5 MB PDF often becomes a 9 MB merged file instead of 10 MB, though the exact savings depend on how much the documents have in common.

If file size matters (for example, because you need to email the document), merging actually helps. If the merged file is still too large, you can compress it using a separate tool, but that usually reduces image quality.

Keeping your order straight when merging more than two files

Most merging tools let you drag and drop pages or files to reorder them before you merge. In browser tools, you usually see thumbnails of each file and can drag them up or down. In desktop tools like Acrobat Reader, you add files one at a time and can rearrange them in a list before clicking merge.

The key is to arrange them before you merge, not after. Once the files are combined into a single PDF, reordering pages is harder and usually requires a paid tool or a more technical approach. Take the extra 30 seconds to get the order right the first time.

Frequently Asked Questions

Can I merge PDFs that have different page sizes?

Yes. Most merging tools combine PDFs regardless of page size — a letter-size page (8.5 by 11 inches) can sit next to a legal-size page (8.5 by 14 inches) in the same document. The merged PDF will preserve each page's original size, so when you open it, you may see pages of different dimensions.

What if one of my PDFs is password-protected?

You need to remove the password before merging. Open the protected PDF in Adobe Acrobat Reader, enter the password, then save it as a new file without a password. After that, you can merge it with other PDFs. Browser tools usually cannot handle password-protected files at all.

Does merging PDFs change the quality of the images inside them?

No. Merging combines the files without re-compressing images or text, so quality stays the same. The only exception is if the merging tool has a compression option that you turn on — most do not by default.

Can I undo a merge after I have saved the file?

No. Once you save a merged PDF, the original files are separate from it. If you need to separate them again, you would have to use a PDF splitter tool or manually recreate the original documents. Keep your original files until you are sure you do not need them.

Is it safe to use a free online PDF merger with confidential documents?

It depends on your risk tolerance. Browser-based tools upload your files to their servers, which means a company you do not fully know has temporary access to your documents. If the information is sensitive — medical records, financial statements, legal documents — use a desktop tool instead so your files never leave your computer.