If you are just showing PDF files you could get away with something like an Amazon Kindle Fire 8 inch. They aren't the fastest things, and you might get some stutter if these PDFs are big files, but the screen is good and the size is just right for easier reading. They are also pretty darn cheap at right around $100 USD.
If you are looking for something faster you could look at Samsung's Galaxy Tab line-up. They also have an 8 inch tablet that has a nice screen and is pretty quick. That'll probably be around $160 USD.
Finally, if you are even considering giving your employees tablets to replace any laptops, still be nice to show PDFs on, and can run programs like Microsoft Office, you should consider the new Microsoft Surface Go. They...