The moment it fails
The download of a contract, an invoice or a course book stopped, and the reader now says the PDF is damaged or cannot be opened. Sometimes it opens blank. The pages that arrived are often still in the file.
Why the reader refuses it
A PDF is a set of numbered objects, pages, fonts and images, with a cross-reference table near the end that says where each object starts and a trailer that names the document's catalog. A download cut short loses that end first, so the reader cannot find the pages even when most of them are there. When the start is lost, the header that says the file is a PDF goes with it.
What happens to the file
The tool scans the file for the objects themselves, rebuilds the cross-reference table from where they really are, and writes a new catalog and page tree when those were lost. Pages whose objects arrived whole are kept as they were; a page that is missing pieces is left out rather than passed on broken.
What you come away with
A PDF that opens with the pages that arrived. If the download stopped at page 40 of 60, expect roughly the first 40, minus any page whose images or fonts were still to come. Text, images and fonts are not redrawn or re-encoded.
Where it works, from our tests
A PDF cut short, and one whose start was lost, came back in every case we tested. A damaged zone in the middle, shifted data and scattered errors usually left the file as it was: in those cases a new copy from the source is the way back. The file types guide has the full table.
Before you start
Keep the original. If the link to the PDF still works, downloading it again is quicker and gives you every page. A PDF protected by a password still needs that password to be read afterwards.
Check the new PDF
Page through it to the end and compare the page count with what you expected. Look at the pages you need most first: the signature page of a contract, the totals of an invoice.
Common questions
Will the whole PDF come back?
Only the pages that arrived. A page that never reached the file cannot be rebuilt.
Is the text or the layout changed?
No. The pages kept are copied as they were; only the tables that tie them together are written anew.
Is the PDF uploaded anywhere?
No. It is read in your browser, on your device, and never leaves it.
Sources
Documents consulted for this guide.