Linux

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.