I appreciate your reply!
This “Dangerzone” is still foreign to me. My primary concern with finding a PDF reader that was the best for preserving user privacy was that the PDF reader App would send telemetry back to someone on the Internet that would be able to see every PDF file a user opened.
I’m not concerned about opening a malicious PDF file, instead, I was concerned about the PDF reader App sending telemetry back to the PDF reader App author online.
We have recommended many apps that dont send telemetry (including Okular).
Has no one mentioned MuPDF Viewer? Minimalist PDF reader for Android that doubles as an ebook reader.
Just block it, in the firewall?
Which firewall?
I’ve been having endless problems with Windows 10’s inbuilt firewall so I’ve been looking to setup a 3rd party software firewall made by another company.
safing portmaster, though doesn’t work for windows ARM yet.
Probably don’t grant sensitive permissions to a random 3rd party firewall. While I agree with you that Windows firewall is pretty decrepit, it is much better to use the OS’ tools rather than rely on a third party. Probably look up a guide for this.
Is Portmaster still the way to go for a Windows firewall?
Well I use the built-in Windows anti-virus as it seems to work really well but the Windows firewall seems like it’s low on security features and reliability… 
I appreciate your reply!
Do you think the use of a sandbox tool would be helpful? Such as this: https://dangerzone.rocks/
I don’t have much experience with sandbox tools so any replies would be great!
Maybe an atypical recommendation but something with no Wifi, with E-ink and quite comfy on the eyes to read on would be a Boox Page.
It’s not too expensive (paid 200€ for it), lasts for an entire month and is old-school “drop the file on an SD card” kind of device with no BS service that could nuke the device out of nowhere. 
Is it the most privacy-focused device? Probably not, but it’s a Linux device that never connects to the Internet so that’s good enough for me. Having the thing dumb is quite positive in this case.
No one mentioned Only Office which has a great PDF editor / reader.
Just block internet connections in your firewall after downloading it, as you’ve mentioned.
The problem with Okular on Windows is you need the Windows Store.
I got so used to SumatraPDF on windows that I tried to use it in Linux via wine (crossover), but it’s not good that way. It’s a pity it’s windows only, but it’s a great piece of software, just like it should be
I take it that, even after Foxit PDF Reader fixed all of their vulnerabilities, that it is a no go? What about PDF XChange Editor?
@jonah I think you should merge this thread with that other thread for pdf editor recommendations
I appreciate your reply!
How is this a problem? I believe the Okular website recommends downloading from the Microsoft Store… 
All platforms support PDF.js in the web browser. Alternatively:
Evince is indeed very basic but works quite well!
I discovered sioyek recently too, quite powerful and nice if you learn how to use it.
Agreed for the rest too but I have no clue for Windows. 
I appreciate your reply! I’m just not sure how sioyek is a better PDF reader in terms of user privacy and user security over the others like MuPDF and SumatraPDF???
I appreciate your reply!
I didn’t realise SumatraPDF was held in such high regard!
I’m still trying to wrap my head around https://dangerzone.rocks/
How does software like Dangerzone make PDF files safer to read?
More Microsoft telemetry. I removed the Windows Store on my device.