I've only just managed to try uploading from a different computer - and the upload was fine.
Thanks for the info re my computer. I've tried renaming - in all instances it gives the file extension as .pdf prior to upload; intended uploads always open as pdfs when I check them on my computer just prior to uploading; the asset library interface gives .pdf as the upload file just before I press 'Save and Publish.'
My computer seems to changes its mind at the very last moment, lol!
The files upload as pdfs elsewhere on the cloud; I can host them elsewhere and hyperlink.