The fastest way depends on what you're looking for
If you need to spot differences between two PDFs, your best route depends on whether you're checking for small text changes, layout shifts, or just want to read them together. Built-in tools in Adobe Reader and free online services handle most everyday comparisons without installing anything new. For detailed line-by-line work — catching every word change or tracking edits — dedicated comparison software shows differences in color-coded highlights.
The method you choose also depends on file size and sensitivity. A two-page contract is fine to upload to a web tool. A 500-page medical record or confidential document should stay on your computer, which means using desktop software instead.
Key Takeaways
- Adobe Reader's side-by-side view lets you open two PDFs at once without any special tool, though it shows them as separate windows rather than merged.
- Free online tools like Diffchecker and SmallPDF let you upload two files and see differences highlighted in seconds, but avoid them for sensitive documents.
- Desktop software like Beyond Compare and WinMerge shows every text change in color-coded detail and keeps files on your computer.
- For layout-heavy documents like design files or scanned images, visual comparison tools work better than text-based ones.
Using Adobe Reader to view two PDFs at once
Adobe Reader (the free version) lets you open two PDFs in separate windows and position them side by side on your screen. Open the first PDF, then go to File > Open and select the second one. Both files will open in the same application, and you can drag the windows to sit next to each other or use your operating system's snap feature (Windows key + arrow on Windows, or Mission Control on Mac) to split the screen.
This method works well if you're reading both documents and spotting obvious differences yourself — a signature that moved, a paragraph that vanished, a date that changed. It does not highlight differences automatically, so you're doing the visual work. For contracts or forms where you know roughly what changed, this is often fast enough.
Online comparison tools for quick checks
Web-based tools like Diffchecker (diffchecker.com), SmallPDF (smallpdf.com/compare-pdf), and iLovePDF (ilovepdf.com) let you upload two PDFs and see differences highlighted automatically. Upload both files, click compare, and the tool shows you text that was added (usually in green), removed (usually in red), or changed. Most of these services are free for files under a certain size — typically 50 MB or less.
The trade-off is privacy. Your files travel to someone else's server, so do not use these tools for documents containing passwords, medical information, financial details, or anything confidential. For comparing two versions of a public document or a draft you're working on with a colleague, they're fast and require no installation.
Desktop software for detailed text comparison
Beyond Compare (a paid tool, around $30 one-time) and WinMerge (free, Windows only) are designed specifically to show every difference between two files. You point them at your two PDFs, and they display the text side by side with every addition, deletion, and change highlighted in different colors. You can jump from one difference to the next, ignore whitespace-only changes, and even merge edits if you're combining versions.
These tools work best when PDFs contain mostly text — contracts, reports, legal documents. They struggle with scanned images or heavily formatted design files because they're reading the text layer, not the visual layout. If your PDF is a scanned image with no text layer, these tools will not be able to compare it at all.
The advantage is that files never leave your computer. Everything happens locally, which matters if you're comparing anything sensitive or proprietary.
Comparing scanned PDFs and image-heavy documents
If your PDFs are scanned pages or mostly images, text-based comparison tools will not work because there's no text for them to read. Instead, you need a visual comparison tool. ImageMagick (free, command-line tool) can overlay two images and highlight pixel differences. Pixelmatch
For a simpler approach, open both PDFs in an image viewer or design tool like GIMP (free) and flip between them, or use your operating system's built-in preview tools to zoom in on specific sections. This is slower but requires no new software.
When to use each method
| Situation | Best Tool | Why |
|---|---|---|
| Two versions of a contract or report | Diffchecker or SmallPDF | Fast, automatic highlighting, no installation needed |
| Sensitive or confidential documents | Beyond Compare or WinMerge | Files stay on your computer, no upload required |
| Scanned pages or images | GIMP or visual flip-through | Text-based tools cannot read scanned images |
| Quick visual check, no highlighting needed | Adobe Reader side by side | No extra software, works with any PDF |
| Detailed line-by-line comparison with color coding | Beyond Compare | Shows every change, lets you jump between differences |
Frequently Asked Questions
Can I compare PDFs on my phone or tablet?
Yes, but options are limited. Most phones do not have desktop comparison software. Online tools like Diffchecker work in a mobile browser, though uploading and viewing results on a small screen is awkward. For serious comparison work, a computer is more practical.
What if one PDF is a scanned image and the other is text-based?
Text-based comparison tools will only see the text in the second file and miss everything in the scanned image. You'll need to use a visual method — opening both in a viewer and checking them side by side — or converting the scanned PDF to searchable text first using OCR software like Adobe Acrobat or free tools like Tesseract.
Do I need to buy Adobe Acrobat Pro to compare PDFs?
No. Adobe Reader (free) can open two PDFs at once for visual comparison. Acrobat Pro ($13 per month) has a dedicated compare tool that automates highlighting, but it is not necessary for basic comparison work.
Why does the online tool say my files are too large?
Most free online comparison services limit file size to 50 MB or less to keep their servers from getting overloaded. If your PDF is larger, use desktop software like Beyond Compare or WinMerge instead, which have no file size limits.
Can I compare more than two PDFs at once?
Most tools compare exactly two files. If you need to check three or more versions, compare the first two, then compare the result against the third. Beyond Compare can handle multiple files in some workflows, but it is not designed for comparing three PDFs side by side at once.