SteveS When I go the the Messages app and tap on the little paper clip, it's not there.
When you attach a file to the message, it retrieves files from the Recents section of the Files app I believe. For whatever reason, only images (and maybe videos) show up there. Assuming you're using the default Messaging app, on the "image" tab of the attachment options click the green box to open Files, then the "hamburger" (three horizontal lines) in the top left next to Recents and navigate to where the file is located and PDFs will show.
However, with that being said, it doesn't seem like you can attach PDFs. At least, I tried and the PDF is greyed out and unable to send.
I can suggest that you use something like Send to transfer the file to your friend with a link in a message.