PDF files on Google Drive not recognised as PDF

I’ve encountered a strange problem with PDF files stored on my Google drive. Hopefully someone can come up with an explanation. I have a number of scientific papers stored on my Google Drive as PDF files. Some (but not all) were not viewable in the drive as they were not being recognised as PDFs although they contained a .pdf suffix. I could download them, however, and open them in Preview. If I then did a Save as… from Preview and uploaded the re-saved files back to the Google drive they were all now recognised as PDFs and could be viewed in the drive. Any explanation?