Unable to view comments in attachments

Hi,

I'm working with someone on a document. However, when he sends me his PDF, some of his comments aren't viewable. In particular, there are areas where he replaced text with something else, but on my end it only shows as the text being crossed out (it doesn't give what was typed to replace it). Does anyone know what would cause this? We're both running Adobe Acrobat Pro DC 15.020.20039. I did do a search for possible solutions, and I heard that it could be caused by the file being flattened. However, no one flattened it.

Update: I don't know if this is relevant, but the person who sent me the PDF is using a PC and I'm on a Mac. I tried deleting and redownloading the file, and now I can see the comments. I still don't know why it wouldn't show in the first place, though. Any ideas?