Before getting started, note that there are several scam PDF viewers in the Microsoft Store, with fraudulent ratings and laced with ads and pop-ups. Shame on Store Q&A yet again. In the meantime, stick to AAWP's links below and you won't go far wrong.
I've also stuck to UWP apps - I know that there are some old Windows Phone 8.1 viewers that still work, but I do try and only use native Windows 10 (UWP) applications on my phones and I'm pretty sure you'll be the same.
Built into every Windows 10 Mobile (native or upgraded) phone, of course. I'm testing this on a Lumia 950 and Edge has been reading PDFs just fine for a while now. There's no easy* way to open a local document from Edge's menus, but if you use the supplied (in the OS) File Explorer utility then tapping on a PDF file in your user folders will result in it being loaded up into Edge (yes, even if you have other PDF readers, as here, installed).
* In fact, you can do this, but only with lots of convoluted typing, i.e. constructing a 'file:///' style URL!
As you'd expect when retrofitting PDF reading into a web browser, as Microsoft did (the impetus was apparently its ill-fated drift into using Edge for e-reading), the interface doesn't change much and you don't get many bells and whistles. However... it's reasonably fast (renders of pages within very large documents can take a couple of seconds, but it's easily bearable), there's an easy 'go to page' number box (top left), and the search function is very fast and easy to use.
Pinch zooming in and out is intuitive and responsive. Overall, top marks apart from the file opening double act with File Explorer! (It goes without saying that if you encounter PDFs online then Edge handles them 'inline', as it were!)
This has it all and is free as far as I can see (the developer does a number of other commercial PDF apps for company use), though it's not the fastest at rendering PDFs from the options here. Zooming, navigation, searching, it all works very well, if slowly for longer documents. Although there's no page number 'goto' function, the always-accessible thumbnail 'Pages' view has page numbers, so it's still easy to get to a specific place in a document.
Terrific software, though again I have to point out that it has got slower with updates over the last two years and I wonder whether the developer has the will to do some optimisation. Note also that this application works for printing and searching XPS, OXPS and text files, EPUB, MOBI and FB2 eBooks.
Another full UWP solution for all Windows 10 devices, this is fast and powerful. It's more than just a reader too - though it does have search, zoom, navigation and other essential reading functions, as you can annotate PDFs too. Amazingly full featured considering the price.
Another one to definitely try, though the interface is at times wonderful (dark theme - including document pages(!), moving through search results and previews) and at times quirkily annoying (general use of white space and panels). A very different approach to a user interface.
When I first tried this app a couple of years ago it was very unstable and kept crashing - it's quite a bit better now, though I still had the app freeze up on me when asking it to open two PDFs at the same time.
This tool is a good, fluid, and very fast option, especially if you have a powerful W10M phone. It's rather over the top for the use case on this page though, and the full range of editing features aren't needed here, though toggling on the edit mode with a top-left tap is the way to access the 'go to page' jump field. And then toggle the mode back again for straightforward browsing.
The interface is again a little odd in places - coders go out of their way sometimes to avoid using standard interface conventions! Most curiously, I couldn't find a search option - in long PDFs it's the best way to find things. Am I missing something, Roxy?
There's a free non-Pro version as well, with adverts, but as the Pro version is on sale at the moment, at only a few quid, I'd like to see the developers, Roxy, get some sales and earn them a beer or two for their hard work!
This works really well as a simple PDF browser - it's fast and easy to adjust views, jump around, and so on. I particularly liked the thumbnail view, letting you get to where you want to go graphically if there's a particular image or element that you're looking for.
On the downside, this hasn't had any updates since my first article, and so I still couldn't get the search function working, but that's a small point for many documents. Well worth a shot, considering the price (free).
Another great option with hooks through to a more extensive online file exporting and conversion service, this offers searching, thumbnails and navigation, annotations and content copying, a dark theme for content as well as UI and, somewhat experimentally, a 'reflow' mode, which comes in handy when trying to read A4-formatted pages on a small phone screen.
Foxit has been around for years, but the UWP version for Windows 10 is well done and also well worth a look if your PDFs aren't too huge (reflowing the book-size test PDF was beyond it). Foxit says that the app has been replaced by 'PhantomPDF', seemingly a complete rewrite, but this requires a version of Windows 10 that's beyond where Windows 10 Mobile ended. So we're stuck with the slightly buggy app here. Still worth a try though, as it's slick and fast.
Notably, all of these solutions are either free or with a free version, so there's zero excuse for trying them out and seeing which one suits your PDF documents and needs best.
PS. If you're wondering about the 'Elop' PDF being tested/viewed here, it's this. A cracking read!