Currently the default PDF app i.e. Pdf Viewer Plus lacks a very essential feature: Text Search; which is a pinned issue on the github page of the app. It seems like to implement the search feature the library has to be changed which is a tedious job.
My suggestion is to ditch this app in order to adopt an already mature and much robust MuPDF. It works flawlessly and it has Text Search feature among others like Annotation and Filling out forms etc.
I’m not affiliated with either of those projects, just pointing out a very practical problem with the existing default app and suggesting another FOSS app that doesn’t suffer from the same issue.
I had my own issue(1) with pdfviewer plus, but different users look for different features, a feature matrix can help drive discussion? searching is a good use case, but if you open a backlog issue, I think the comparison needs to encompass all available features.
Default app discussions are alive and well, you can find the old mupdf complaints from 2019, but they concerned mupdf mini if I see correctly
(adjacent, but also search: upstream Lineage Eleven the music App has got search removed a few versions ago)