I think this is an excellent suggestion. I used Cloudflare tunnels until recently, and it was very effective. However, I stopped because of a minor issue, which I'll mention in case its a deal breaker for anyone.
Technically, using Cloudflare tunnels for Jellyfin is a ToS violation. You're only allowed to do so if you have an enterprise account, which is quite expensive.
I heard from a "friend of a friend" that everyday users don't need to worry about this. Cloudflare are aware of people using tunnels with Jellyfin and they aren't fussed. The rule is supposedly there to combat large scale piracy.
However, I have heard that cloudflare does decide to start caring if they can use jellyfin use as an extra excuse to kick anybody involved in other ToS violations.
In all likelihood, this won't be a problem for you. While I used tunnels, they worked perfectly. However, given that you are going to go to the effort of sorting out some level of infrastructure for yourself, its something to keep in mind.
I use Adobe Acrobat professionally. I am also on Linux as a daily driver.
Bad news, it's very very difficult.
If your gf's use case involves sharing PDFs with other people at scale, there just isn't a "low tech" solution. Every workaround I use involves some level of programmatic nonsense. For reading and relatively simple tasks, many of the open source alternatives do wonders, but they aren't able to replicate some of the heavier features. Even when they are, you loose compatibility; something critical in certain workflows.
I am stubborn. I force my gimp files upon my Photoshop using colleagues. I edit videos in Blender instead of Premiere Pro (would recommend). I send open source documents and spreadsheets that work neither with my Google nor Microsoft devoted co-workers. But Adobe Acrobat wins. The FOSS readers are probably better, but for interactive, highly formatted outputs you can't get around it.
I have a lonely second box at my work that I reluctantly boot windows on. I too would love an alternative.