I've come across two similar issues in the past month, one from my work colleague and one from my wife.
First one was a Keynote file that wouldn't open "for some reason". I duplicated the file, it wouldn't load. Uploaded it to iCloud, started editing, worked nicely. Exported it out of iCloud as a Keynote file, would load in Keynote, but upon saving it, would no longer load again. That's when I exported it from iCloud as a Powerpoint file, Uploaded it to Google Sheets, exported it out of Google Sheets as a Powerpoint file and loaded that into Keynote and saved that as a Keynote file. That's when it started working. Also exported a PDF version out of iCloud for good measure - just in case something would go wrong, they could at least copypaste the data into a new Keynote file.
Really shook my confidence in Apple.
The second one was just today, my wife called me into the living room going "I've got a problem", and showed me the dreaded "Pages document can't be opened for some reason. - To open this file, restore it to a previous version" -- the previous version list would not load anything. I knew she had been doing this massive 80 page translation from English to Russian on her iPad, so slammed the iPad to Airplane mode, duplicated the document in Pages, enabled Wi-Fi, had the duplicate upload to iCloud Drive and loaded that on her MacBookPro. Got the same error, again, no version data available. Then decided to look at her Pages folder and make a copy of that duplicate. Hey presto! It loaded into Pages without issues. I'm now half-expecting her to save and later load the file and meet with the same "oh you can't load this anymore" issue again, so I guess I'll have to have her CMD-A, CMD-V, CMD-N, CMD-P, CMD-S and hope her confidence in using Apple products hasn't been absolutely destroyed.