Issues with loading PDF Files

I’ve just come across a problem when loading PDF files into media library and then inserting a link to them in posts. What happens is:

• I can use the media uploader to load the files as normal in to the media library. But when I try to view them in the media library a new window opens with an error message “403 access is forbidden you do not have permissions to view these files.”
• So I tried inserting the file into a post as a downloadable link, saved the post and then published it. When I view the post and download the file it appears with an unusual format. Instead of “filename.pdf” it shows as “filename.pdf.html” When I click on the downloaded file in my browser I get the 403 error message again.
• I’ve tried this on both my websites https://www.davidfernley.com and https://www.ryedaledogrescue.org.uk and I get the same problem. I’ve tried it with all plugins disabled and enabled and still had the same issue. I cleared my caches and used Safari and Firefox to see if it was a browser problem, but got the same results. I also uplaoded a file and created a post on my iphone, but still got the same results.
• I thought th PDF file I was trying to upload might be corrupted so I created a new file from scatch and this didn’t help. The strange thing is that I can upload some PDF’s from my computer and they work OK, but some don’t. Any new PDF’s I create from scratch plus some one’s I created months ago fail, but others are fine.
• Images upload with no problems, it just seem to be an issue with PDF’s and word documents.

Sorry for the long post, but this is puzzling me. I uploaded some PDF’s a week ago and they worked fine, so it’s not a longstanding problem.

I hoe someone can help. Thanks

Dave