Allow the user to change the default behaviour so that when an internal link to a pdf attachment (in the current vault) is clicked it opens in the default system pdf viewer and not the Obsidian pdf viewer.
The main use case is on mobile devices where there are pdf viewers that make better use of the small screen than the Obsidian viewer.
Also, Obsidian on mobile crashes if a pdf is too complex. This is a big issue for the latest v3.2 Onyx Boox devices. See bug.
A setting to treat files with the .pdf extension like any other file attachment.
(One of the attractions of Obsidian is that the attachments are stored in the file system. This means you can use any app you want with them. Forcing the user to use the Obsidian pdf viewer seems to go against the design principles of Obsidian (Just my non-expert opinion). )
Don’t use the mobile version of Obsidian.