I have a PDF that was created from assembling multiples PDF files.
IF I re-open it later and replace some pages by a new PDF file or insert some PDF pages from a new PDF file something weird happens.
If I carry on working and replace/insert several pages and do some other operations (like editing content), whenever I eventually save all those newly placed/inserted pages become blank.
Here is the only way I found to prevent this from happening;
When I do an insert or replace operation, I have to save right away and quit the document. When I reopen it, my pages are preserved and I can now edit them as I want, they are solid.
And this has happened with many different PDF files, so it is not just one particular file that might be corrupted. It has also happened to a colleague of mine working on a totally different machine and with totally different files. In his case, they were all files he had himself created.
The only common point is that for both of us these are PDF files that had been generated from Word, either via the Acrobat Toolbar within Word or the Adobe PDF printer driver (CTRL-P).
I do not think this is normal behaviour.