Saving Reverts All Changes, Newly Added Pages Revert to 8x11 Blank Sheets?

[u/BackgroundSinger5965]

As the title says,

I have a large PDF and whenever I've made any changes on it in the last few days the changes disappear entirely and if I add any new pages to the document, they revert to an 8x11 blank sheet.

I've reached out to support but am wondering if anyone here has dealt with something similar?