"The document could not be saved: a file error has occurred"

Upon trying to save an extensively annotated PDF in Acrobat Reader DC (latest release), I received the above error message. I tried to save as, both to replace the existing file, then in another location, to receive the same message. By now, the text had vanished from the visible document and only the underlining and comments were visible. I've basically had to re-type all the comments separately in Word, and will now have to correlate the PDF page numbers to the documents page numbers etc etc, an extremely frustrating end to hours of work.

My question is: why has this happened and, given that this has happened on several occasions in the past (but on less important PDFs), how do I prevent it from happening again? I've since reopened the PDF, commented and saved, and it works fine, so this is clearly a sporadic and random bug. Several others seem to have had this happen on the forum but I haven't found a satisfactory fix (or it seems even a proper understanding of the problem) from Adobe. There are no security controls on my PDF. I am running OS X 10.13.3.

Thanks.