How to fix a PDF that won’t open

You click a PDF and get an error, a blank page, a spinning wheel, or a document that stops at page ten. Before you assume the file is lost, know that most “broken” PDFs are broken in small ways, and several simple checks come first. This guide walks through them in order, explains why PDFs break, and says plainly what a repair tool can and can’t get back.

What you are seeing

Different symptoms point to different causes. Match yours to the closest row:

Quick checks first

1. Try another reader. Open the file in a browser if you were using a desktop program, or the other way round. Strictness differs, and one of them may cope.
2. Try another device. If it opens on your phone but not your laptop, the file is probably fine and the problem is the reader.
3. Look at the file size. Compare it with what the sender says, or with the size shown where you downloaded it. A big gap means it was cut short.
4. Download it again. Use a different browser or a wired connection. Interrupted downloads are the most common cause of damaged PDFs.
5. Ask the sender to send it again, or to attach it instead of linking it. Email systems occasionally mangle attachments.
6. Check where you opened it from. Opening a PDF straight from an email, a zip file or a sync folder that hasn’t finished can give you half a file. Save it to a local folder first.

Why PDFs break

It helps to know what is inside the file. A PDF is a set of numbered objects: pages, fonts, images and drawing instructions. At the end of the file is an index, the cross-reference table, that says how many bytes into the file each object starts. A short closing section points to the index and to the document’s main object, the catalogue.

Readers open a PDF by jumping to the end, finding the index, then fetching objects as needed. So the most delicate parts are exactly the end of the file and the index. If a download stops early, both are missing. If a save is interrupted, the index may describe a file that doesn’t exist yet. If a program miscounts a byte, every object after that point is mislocated, and a strict reader gives up even though all the content is intact.

This is why repair works so often. The objects are usually still there. A repair tool ignores the broken index, scans the entire file to find each object, rebuilds the index and the page list from what it finds, and writes a new, consistent file.

Bad downloads and attachments

Most damage happens in transit. A few habits that help:

• Don’t open the file while it’s still downloading. Wait until the browser says it has finished.
• If your download was blocked, resumed or restarted, discard it and start again.
• Some security software rewrites attachments. If an attachment is always broken from one sender, ask them to send it as part of a zip file, or by a file-sharing link.
• If the file arrived by a messaging app, ask for the original, as some apps recompress files.
• Files copied from a failing USB stick or hard drive can have bad sections. Copy them again, or from a backup.

Repairing the file

If a fresh copy isn’t possible, Repair PDF will try to rebuild what is there. It works in your browser, so a confidential or damaged file isn’t sent anywhere. Here is what to expect:

1. Choose the file. The tool checks whether your browser can open it, and whether it starts like a PDF.
2. Press Repair PDF. There are no settings. It rebuilds the index, the page list and the catalogue, and recovers the objects it can read.
3. Read the report. It lists each problem found and whether it was fixed, how many pages it recovered out of how many it expected, and anything it could not recover.
4. Check the preview and download. Look at the first page, download the file, and open the result in your usual reader. Scroll through every page before you rely on it.

Your original is never changed, so there is no harm in trying.

What repair cannot do

Honesty matters here, because a repaired file that looks fine can still be missing something.

• It can’t recover bytes that were never saved. If a 40-page file was cut off after page 28, the other twelve pages are gone. The report says which pages were lost.
• It can’t fix corrupted data inside an image or font. You may lose one picture, or see odd characters, while the rest recovers.
• It can’t guess passwords. Encrypted files need Unlock PDF with the password you hold.
• It can’t keep digital signatures valid. Rebuilding changes the bytes a signature covers, so treat a repaired copy as unsigned.
• It can’t turn another file type into a PDF. A renamed Word file or a saved web page has no PDF in it to rebuild.

Special cases

Very large files

A file with hundreds of megabytes of scans can fail simply because the device runs out of memory. Close other tabs, or use a computer. If you only need part of it, open it on a computer and use Extract pages to save what you need.

Scans that open but look wrong

Pages showing as black squares or garbled images are a rendering problem, not necessarily a damaged file. Try a different reader. If the scan is large, Compress PDF can produce a simpler copy.

Forms that won’t fill in

Some forms use a technology called XFA that only a few programs support. If a form opens as a message saying the file needs a newer reader, ask the sender for a standard version.

Files from a phone

Documents saved from mobile apps sometimes use unusual structures that desktop readers dislike. Open the file in a browser, or print it to PDF to make a clean copy.

Stopping it happening again

• Save attachments to a folder before you open them.
• Let downloads finish, and compare the size if a file seems short.
• Keep the original when you edit a PDF. Save changes as a new file.
• Back up important documents to a second place.
• When you create PDFs, close the writing program properly before you move the file.