Edit: here it is in action, with selectable text: https://www.joachim-breitner.de/various/pdf_screenshot_3.pdf
Wow.
I want to click at the UI in the PDF :( it's not working :(
I'm surprised it's not more popular though, but I guess it has remained undiscovered like it was for me until now, or it's simply not as useful to others as I would think. Maybe because not all apps are GTK3 apps, or people don't want pdf's because they can't be inlined on a issue ticket like plain old images can, for example?
Well, this module can also produce SVGs, which, I guess, can be inlined in some issue trackers. But, aside from being GTK3-only, the main reason this sort of thing didn't become popular is simply due the fact that even JPEG screenshots are just good enough and get the job done. Virtually nobody has any need to search for text in older screenshots; selecting text—well, you can't copy-paste from most of real error windows, so you're going to re-type its content anyways; and nobody is bothered with other people's font sizes, aliasing preferences, JPEG artifacts or whatever.
Aside from that: jpeg with compression artifacts works well enough, so that even png, giving better quality, often isn't used by non-technical folks.
And unrelated as an anecdote: The IT hotline at a bank where I once worked asked for screenshots as Word documents (my assumption: "everybody" knows Word and so teaching "press 'Print' on your keyboard and then paste into Word" is relatively reliable teachable via phone ... this went to some level where we as developers got screenshots of sessions containing customer info, where customer details were "hidden" using black bars from Word, which of course could have been removed easily)