If I create a new note, then attach a PDF file, the note is created and shows a PDF icon. However, the OS does not treat the file as a PDF when opening (I am presented with non-PDF apps to choose from to open the file).
If I instead use the Android share function and share a file to the Joplin app, a new note is created and the link within the note is handled correctly as a PDF.
I have had similar experiences with other types of attachments in the android client. I think it is perhaps adding the links to the files, rather than the content as an attachment. At least, when I sync to other devices, that appears to be what is happening. The original attachment can be opened on the android client, but only get a link on the synced devices.