One job, no settings: the last page becomes the first, the first becomes the last, and everything in between flips accordingly. It exists because doing this by hand in the reorder tool means typing out every page number backwards, which is tedious at ten pages and unrealistic at two hundred.
Why documents come out backwards
Nearly always a scanner. Sheet-fed scanners take a stack face-down and pull from the bottom, so a document placed in reading order comes out reversed. Some multifunction printers do the same when scanning to email. It is such a common failure that many scanner drivers have a "reverse page order" checkbox — which people find immediately after scanning two hundred pages without it.
The other frequent cause is a merge done in the wrong sequence, where a set of single-page files was combined in whatever order the file picker produced.
What it does not fix
- Pages that are upside down. That is rotation, not order — use the page rotator, which can do 180° on every page at once.
- Duplex scans where the backs are all at the end. Reversing makes that worse; the interleave tool is what merges two one-sided passes back together.
- A few pages out of place in an otherwise correct document. Reversing flips everything, so use the reorder tool for a partial fix.
- Right-to-left documents that are correct as they are. Arabic and Hebrew PDFs often look backwards to readers expecting left-to-right and are not.
Completely lossless
Reversing copies the existing page objects into the opposite sequence. No content is re-rendered, re-compressed or altered in any way. Text stays selectable and searchable, scanned images keep their exact original bytes, and the file size is essentially unchanged. Reversing twice gives you back the document you started with, byte-for-byte in all the ways that matter.
That last point is worth knowing if you are unsure whether reversing is what you need: there is nothing to lose by trying it, because the operation is perfectly undoable.
The usual caveats
Bookmarks are not carried across, and form fields lose their interactivity. Password-protected files are refused with a message explaining what to do rather than an obscure error. Everything runs in this browser tab, so a backwards scan of something confidential does not need to be uploaded to anyone to be put right.
Check the first and last page
The quickest way to confirm the result is right is to look at the first page and the last. If the original ended with an appendix and the reversed version starts with it, the operation did what you expected. It takes a few seconds and catches the case where the document was not actually backwards to begin with.