The information dictionary
Every PDF carries a small structure holding Author, Title, Subject, Keywords, Creator and Producer, plus creation and modification timestamps to the second. The author field is filled in automatically from the account name of the machine that made it, which is why it so often names someone who is not the author at all.
Earlier drafts that were never removed
A PDF can be updated by appending changes rather than rewriting the file, which keeps the previous state inside the same document. This page counts the end-of-file markers and reports when more than one is present. That is a structural signal that earlier states exist, not a claim that any particular text can be read back.
Things that are not pages
PDFs can carry embedded JavaScript that runs when opened, complete files packaged inside them, and actions that ask to launch a program. All three are legitimate features and all three have been used to deliver malware. They are detected here by reading the file as data, so nothing inside the document is executed.
Reviewer comments survive
Annotations added during review stay in the document whether or not your viewer chooses to display them. Forwarding a reviewed PDF forwards the review with it, which is a common way for internal remarks to reach an external recipient.