1: make sure you are logged into the Amazon acount that you set up your kindle with
2: make sure you are connected to the same network
3: make sure you know where you downloaded the PDF
Sending the file
go to https://www.amazon.com/sendtokindle. Click "select file from device". select
your file then press "open". make sure that the "add to library" switch is selected
then press "send". Now just wait, it could take a long time.