You can install the Software Development Kit (SDK) for your platform of choice (for example, the Android SDK, iOS SDK, or Windows Phone SDK), which comes with an emulator for that phone platform. (Note: iOS SDK is only available on Mac.)
You could also open the PDF on your computer and zoom out until it's the same size as a phone screen, but as Bob mentioned, neither of these options will give you the same experience as using apps on the phone. The only good way evaluate the experience is to actually try viewing the PDFs on a phone. If you don't know anyone with a smartphone, you can upload your PDFs somewhere, go to a cell phone store, and download some of the PDFs on the phone. Then you won't have to wonder whether they'll look good on the phone you're looking at. (Personally, I'd recommend something with at least a 4" screen.)
That said, you probably won't be happy reading PDFs without reformatting them for your mobile device, simply because an 8.5x11 sheet of paper shrunken down to a roughly 4" diagonal page are going to be too small to read comfortably without zooming in...and zooming and panning to read a page is annoying.
But fear not! As Bob mentioned in a comment, PDF reader apps can often automatically "reflow" text to fit the screen better. You can also use Calibre to convert many file types into any of various eBook formats. It works really well for books that are mostly text, but can have less-than-optimal results if your eBooks have a lot of tables or images.
One more thing worth noting: if you primarily want a portable device for reading, you should also consider getting a reader with an e-ink screen, like the Kindle or Nook. It takes a few days to get used to the way the screen flashes on each page change, but the reading experience is very close to reading a paper book. They're bigger than phones, but still quite a bit smaller than tablets. I use Calibre with my Kindle Keyboard and highly prefer reading eBooks on the Kindle over reading them on my smartphone or tablet.