If you have the setting to "Open PDFs from last session on Acrobat launch" enabled, but Adobe Acrobat has updated since its last launch and attempts to pop up a new feature notification WHILE it is opening your PDFs from the previous session, it can trigger an error that causes all the rest of your previous-session PDFs to fail to open. Specifically, an error stating "There was an error opening this document. Acrobat cannot open this file because a task is still active in Acrobat. Please return to Acrobat and end the task before opening this file." pops up OVER the new feature popup that is CAUSING the error; so you cannot close the new feature popup and can only click "OK" on the error popup, which abandons the attempt to open that PDF. Then, if there were multiple PDFs that Acrobat had not loaded before the new feature popup appeared, the error will immediately re-appear for the next PDF; and this repeats until all of your remaining PDFs have been lost. Only then can you close the new feature notification popup, and closing and re-opening Acrobat does not automatically re-open the PDFs that failed to load, leaving you to go through your files and manually re-open each PDF that was lost (assuming that you can even remember what they all were). This issue occurred to me today (see attached screenshot), and I've also experienced it on one or two other occasions over the past several years. I only use Windows PCs, so I don't know whether this bug exists on Mac. The obvious solution would be to either prevent any new feature notification popups from occurring until ALL previous-session PDFs are finished loading, or to prevent new feature notification popups from interfering with the opening of PDF files.