What this can fix: a corrupted file header, a broken or stale cross-reference index, a damaged trailer, and junk bytes left after the end of the file - the usual damage from an interrupted download or a crashed editor. The document is rebuilt around whatever page objects survived.
What it cannot fix: content whose bytes are genuinely missing. If a file was truncated mid-transfer, the lost pages are gone, and modern PDFs that store their pages inside compressed object streams may be unrecoverable once that stream is damaged. You will be told clearly rather than handed a silently broken file.