In iOS 13, I believe the only way to attach a PDF in your Files folder is to go into the Files folder, copy the file, and then switch back to iMessage and paste.
I thought in iOS12 you could do this easier by long pressing in the iMessage text box and a pop up would show something like “Add Document” comes up (like it still does in Mail).
Is there a better way to do this in iMessages? Am I hallucinating that this pop up was there before? Thanks!