When you do this Joplin can not detect the attachment has been changed and won't sync your change.
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Joplin uses the default pdf viewer on you OS. You can change it in the OS settings, i don't know what OS you use so I can't help beyond that, but you can just search online for how to do it.
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I know that, and that's ok. It's just a bit strange having to change the default app first when I want a PDF-editor to open a PDF instead of the default PDF-viewer.
I agree it would be useful to have something like "Open with" on Windows. Not sure if it's possible with electron though.
Yes, also on Mac 
I have recently tried to highlight a number of problems with attachments across all platforms - in a post called attachments - in and out.
Some of the problems listed there seem related. Could we merge these threads, so that the devs can find it all on one place should they ever decide to clean it up ?
(@roman_r_m : unfortunately the recent quick fix of one problem in Android related to attachments didn't work -> see github)
Looked into this and this seems to be a problem with how files are attached on Android rather than the code that shares attachments. For instance, I believe it should work correctly for attachments that were created on desktop and synced to Android.
It also works for attachments created by sharing to Joplin.