No app needed
Most PDF apps for phones can open, sign and annotate a document. Very few can compare two of them, and the ones that can usually want a subscription before they show you anything. You don't need one. The comparison tool on the homepage runs in Safari on iPhone and iPad and in Chrome on Android, with nothing to install.
That matters more than it sounds. The moment you most need to compare two PDFs on a phone is usually the moment you are away from a laptop: a landlord has emailed the final lease, a recruiter wants the offer letter back today, or a client has returned a quote with a note saying they "tidied it up".
Step by step
- Save both PDFs somewhere your phone can reach them: Files on iPhone, Downloads on Android, or Google Drive, iCloud Drive or Dropbox.
- Open the homepage of this site in your browser.
- Tap the Older version box and pick the original PDF. Then tap the Newer version box and pick the revised one.
- Choose Side by side or Redline PDF. You can switch between them after the comparison runs.
- Tap Compare documents. The list of changes appears with page numbers. Tap a change to jump to it.
- Download the redlined PDF if you want to send it to someone.
On a small screen the change list is the part to read first. It shows the old text struck through and the new text next to it, so you can see each edit without pinching and scrolling around a full page.
Getting the files onto your phone
This is where most people get stuck, not the comparison itself. A few things that help.
If one version came as an email attachment, open it and use Share, then Save to Files on iPhone, or the download icon on Android. Attachments you have only previewed are not always saved anywhere the browser can pick them up.
If the earlier version is a Word file you sent from a laptop, open it in the Word or Google Docs app on your phone and export it as a PDF first. The tool compares PDFs, so both sides need to be PDFs.
If the PDF lives behind a link in a signing service, look for a download option before you sign. Comparing before you sign is the whole point.
A worked example
Say you agreed a rental at 18,000 a month with a two-month deposit and a one-month notice period. The landlord sends the final agreement as a PDF and asks you to sign it by evening.
You open the version you last agreed, from the earlier email, and the new one, and compare them. The change list comes back with two entries. On page 2, "two months" became "three months" in the deposit clause. On page 5, the notice period went from one month to two. Nothing else changed.
Now you have a specific question to send back instead of a vague worry. Most of the time a change like that is an old template being reused, and it gets fixed quickly once you point at the exact clause.
What it is good for on a phone
Checking a rental agreement, loan offer or job offer that arrived by email against the version you agreed, before you sign it on the spot. Confirming that a quote or contract a client sent back is the one you sent. Seeing what changed in a revised policy document from your insurer or bank, which often arrives with a line saying "some terms have been updated" and no detail.
For anything longer than a few pages, the phone is fine for the comparison and fine for reading the change list. If you then need to mark up the document or write a long reply, a laptop is easier. The result is the same either way.
Limits worth knowing
Each file can be up to 20 MB, and the pair up to 25 MB. Without an account, documents can be up to 30 pages each. A free account raises that to 100 pages and gives you five comparisons a month.
The tool reads the text inside the PDF. A PDF made by photographing a paper document with your phone camera is just a picture unless the scanning app added text recognition. Many scanner apps do this automatically; some only do it if you turn it on. If a file is refused because it has no text, the guide to comparing scanned PDFs explains what to do.
Privacy
Both files are sent over HTTPS, processed in memory and discarded as soon as the result is returned. Nothing is stored, there is no history of your comparisons, and no account is needed for documents up to 30 pages. The privacy policy sets out what is and is not kept.
If you want to know more about what to look for once you have the result, how to compare two PDF files for changes covers it in detail.
Try it on your own documents. Upload two versions on the homepage. Documents up to 30 pages need no account, and nothing is stored.