The fontsize of the PDF reading interface can not be modulated by the View menu
As we know, the font size of the library interface can be modified through the fontsize option in the View menu, but the fontsize in the panels on both sides of the PDF reading interface cannot be changed synchronously
However, on my 3440x1440 screen, the font size in the annotations and doc outline panes looks to be 8 points. Tiny compared to 14 point size I have set everywhere else.
For finer font control in Windows (especially file explorer), I use System Font Size Changer. It's shareware, minimal cost and one license allows use of all his font sizer tools. Get directly from the developer at https://www.wintools.info/index.php/system-font-size-changer. I've developed software for 20+ years and appreciate a great tool like when I see it - they are rare.
Here is the solution:
1. Go to Edit | Preferences | Advanced
2. Click Config Editor
3. Search for "font.minimum-size.x-"
4. Change the setting that matches your language. For US english, it is "font.minimum-size.x-western"
5. Click OK
What's great is that the zotero window updates immediately so you can keep increasing the size until it's right for you.
Kudos to Zotero!!!