Some PDFs have fonts that are too large to maintain an optimal reading speed. Is there a normal page-scrolling (not flipping) PDF viewer that can stay zoomed out at the zoom level you've pinched to? Everything I have tried so far automatically zooms back to 100% after releasing the pinch-zoom.
MarginNote has a vertical scrolling mode (though it's not the default) and can stay zoomed out at about 50%.
Found this thread, by searching for an answer to the same problem.
By Providence, I did find an alternate app, which accomplishes this ability to "zoom out" (to less than 100%, etc.), on a PDF.
However, it can tend to be somewhat tricky — but not at all impossible — to get the file into the app, in the first place.
(You'll figure it out, if you have enough initiative to download the app, and to tinker around, with it).
Here is the name of the app: "PDF Editor ,PDF Book Reader" by Milad Fakhr
I hope that helps!...