I played around with this and it picks the first pdf in the list of its children. So I figured out a simple work around.

I started off with
AAAAAA
BBBBBB

I can "rename" them as:

(1) BBBBBB
(2) AAAAAA

then I get the one I want. The "name" is just a label. It is associated with the file name of the attached file.

The name shouldn't matter.

Unless the comment is outdated, this is what it does:

Looks for attachment in the following order:

oldest PDF attachment matching parent URL,
oldest PDF attachment not matching parent URL,
oldest non-PDF attachment matching parent URL,
old non-PDF attachment not matching parent URL

I'm pretty sure adding an option for that would create more confusion than it solves.

I think that's the biggest concern with this.

I had the following thought. In the right hand panel we can have a button or a checkbox that says something like "Open this attachment by default". Only one of those check boxes could be checked per parent item of course and, by default, they would all be unchecked.

I can only see this being useful for items that are commonly accessed. It would also prompt for an additional table in the database, so we probably wouldn't see this until some major changes to the DB.

For future users hitting this page, my workaround to this same problem (which seems to be working as of early 2019) is to:

1. In Zotero, expand the parent item.
2. Right click the current default PDF.
3. Choose "Show file", which opens its folder in your file manager (e.g., Explorer on Windows).
4. Using your file manager, copy the PDF file to another temporary location on my PC (I just popped it on my desktop).
5. Back in Zotero, right click the current default PDF and choose "Move item to trash..." and click OK to confirm.
6. (Optional, to check) Double click the parent item in Zotero to check it's now opening the PDF that you want to be default. Once you've checked, close it in your PDF viewer so you can check again at step 8.
7. From your desktop (or wherever you put the temp copy of your 'wrong' PDF file), drag it back onto the parent item on Zotero.
8. Again, double click the parent item in Zotero to check that it's still working as you want.
9. Also check that from the expanded parent item in Zotero you can open the non-default PDF by double clicking it directly.
10. If the check at step 9 shows the 'wrong' PDF is correctly stored in Zotero then you can safely clean up, deleting the temp copy of the PDF from your desktop.

That all seems to work to persuade Zotero to treat the old-default file as a new file, and hence to prioritise the other one.

The solution by jim from 2/9/19 works but yea, I don't see why confusion would be created by adding this funcionality, as described by askfeps. Thanks!

Try this: https://github.com/sharpevo/zotero-pdfkit