Email them to yourself and pick Open in... and select Kindle. Or use any other method (dropbox, etc) that lets you choose to open in another app. You could probably even use something like iExplorer (Mac or Windows) to open the Kindle directory under Apps. I'm not sure which directory it would be and I can't check right now, but I would guess the Documents folder (or a subdirectory in Documents).
1-Email the PDF file to yourself.
2-Click on the link to open it.
3-It will be open in your browser.
4-There will be two buttons:
-Open in iBook
-Open in other : Here you can choose Kindle or any other PDF readers.
5-Your pdf file will be open in your reader app.
PDF in Kindle app doesn't permit highlighting nor commenting
I just added a PDF to the kindle app on iPad, as I got stuck in iBooks due to the terribly missing function of highlighting text and adding notes on a PDF in iBooks. I am so disappointed that the Kindle app doesn't allow these functionalities either on PDF, whereas on .mobi it does an excellent job. Would anyone know of a (free) PDF reader on iPad which allows these? I have Blue fire reader and Stanza installed, but both no luck with PDF either.
You get your own special email address with kindle. It is email@example.com. Email the pdf to that address, and it will appear in your kindle app. I just tried it, and it works like a champ. This was on my ipad.