I just checked, basically you need to connect your Kindle with the cable to the computer and then copy the PDFs into the Documents folder on the Kindle. Then you can open those by going to the Home screen on the Kindle and clicking on 'Documents' or 'Docs'. The PDFs should appear there.
One thing to keep in mind is that some PDFs use text and some use scanned images of text. The latter do not display well on the Kindle.
If all that doesn't work, then converting PDFs to mobi should work, unless it is a PDF with scanned images of text.