PDFs creating temp files and original disappearing

A colleague has lost a few PDFs they’re working on. While using the comments tool on both occasions she pressed Ctrl + S (on a PC) to save the file & a message appeared saying it wasn’t possible to save the file. When she checked the folder, it just showed two .tmp files, which couldn’t be opened – the original PDF had disappeared. The temp files have new names which look like a bunch of numbers and letters, not meaning anything to us. Any idea why that would have happened?