It's very simple, requires a bit of work and ends up not being very functional due to the aforementioned factor that it only works with its own internal PDF reader, which in this case does not save the location where you close the file, very important for long Guides.
But the file is actually in PDF format, go to Edit the specific game and add the PDF just as an Extra program, just entering a specific name for it, for example, Prima Guide.
In Desktop mode it should already appear in the game options menu to be viewed and in BIGBOX mode it will be available in the Pause Menu.
There are alternative and much better methods, but much more laborious, where you use another reader like Sumatra, for example, to open pdfs or crbs as if it were an emulator, and you can even let it open that pdf before the game is launched and on an extra monitor/TV.