SHENZHEN I/O not opening manual (PORTAL?/XDG?)

**Game information**
SHENZHEN I/O, Linux

**Distribution name and version where applicable**
Fedora 30.25 (Silverblue variant) [ostree://kinoite:fedora/30/x86_64/base]

**Flatpak info**
Flatpak 1.4.2
nvidia-430-40
default
host

**Problem description**
Several games within steam do not launch a program when a game calls it. For instance, this issue is when the game Shenzhen is supposed to open the manual, nothing happens. On the main menu is a PDF icon with the label Datasheets. It doesn't do anything when clicking. I have natively installed a PDF reader called Zathura, and have Evince installed with flatpack (org.gnome.Evince).

I tried to figure this out myself, as I need to learn how to troubleshoot flatpaks and containers, but I really don't know where to start. I read about flatpak portals and XDG but still don't understand what needs to be done.

**Does this issue reproduce with native Steam**
Have not tried this in any native Steam account.