I own an old XP-Pen Star G430 OSU Drawing tablet that I use for various applications. It work as an input device that controls the cursor and as such, works almost exactly like a mouse. The tablets work on basically any program and I used my Star G430 as my only input device for at least 1.5yrs doing everything from web browsing, drawing and even gaming.
The main issue to arise would concern the particular PDF reader program you are using. If it doesn't have the ability to "draw" on the documents, there's really no point to getting a tablet.
I get you may not want to invest in a new Surface or touchscreen tablet. But then again, if you're only using the tablet as a PDF editor, I might recommend buying a used tablet with a good screen, anything over 1080p, and editing your PDFs with it instead of using a dedicated stylus-device. You can still operate your tablet and workstation PC/Mac next to each other.... just sync the editable document in a folder within aGoogle Drive/Onedrive/Box/Dropbox and then you have best of both worlds.
I think that directly marking up documents on a screen is superior in EVERY INSTANCE. For pure drawing, sketching, shading, and painting in Photoshop or another artboard-specific program you may find Tilt Sensitivity vs. using a touch-screen tablet is equal. But handwriting - it's just so much more natural when you directly edit.
I would recommend the XP-Pen Artist 15.6 Pro
Drawing Tablet Monitor, If you want an external stylus system . The Artist 15.6 pro offers tilt sensitivity. This is HUGE. It makes it feel like a real pencil. This makes using a tablet worth it.