oh, glad you've done it.
someone down thread mentioned things are better if you go through calibre - you might want ot try that.
you download calibre, and it is easy to set up. easiest if you have a gmail email address, apparently.
then you folow the instructions in calibre (ha a good help guide) to transfer/convert etc.
I haven't put any pdfs on my kindle, only books, but it was easy to use calibre for that (to change format etc)