Sorry, but I don’t understand that. Does Graphene OS have an issue with F-Droid? Or the other way round?
Reasoning for at least some of those apps have been discussed at here:
(Edit: I can understand for browser that no point to use instead bromite, but are those all really system apps, which was reason for not include browser?)
https://forum.f-droid.org/t/why-arent-the-lineageos-or-grapheneos-apps-available-on-f-droid/16590
What comes to that pdf reader, I’ve tested it once and now again. Not very userfriendly.
I opened PDF with multiple pages and could jump from first page to last & viceversa. Previously I could read those pages at between but it was cumbersome. Not just scrolling like with Pdf Viewer Plus even at the time it worked.
But otherwise, securitywise up to date app would always be nice.
Edit2:
GrapheneOS reasoning why it’s not ok for them to include app (camera app in this case):
https://github.com/GrapheneOS/Camera/issues/227
See comment of thestinger
Edit 3: Just saying, I personally don’t get their reasonings. Especially cause most videos I’ve seen about graphene os, users usually use f-droid at main profile. They use that sandboxed google services & play store from different userprofile when needed…
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I’ve just intalled it from APP Lounge.
Conclusion: Secure PDF Viewer is not userfriendly at all. Horizontal page scrolling not handled, … Forget it!
Compared to it, Pdf viewer Plus 3.7 is a must !
What about the security issue and Advanced privacy?
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Thanks. I’ll stick with apps from F-Droid 
For alternatives:
GrapheneOS PDF Viewer is good as long as your WebView is up-to-date, and /e/OS is not up to date there: https://divestos.org/misc/ch-dates.txt
Otherwise these ones usually update quickly: MuPDF viewer, MuPDF mini, Librera Reader
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