A problem. I have lots of PDF files on my Google Drive. I clicked on Import on the Kindle in Mobilesheets. I selected Google Drive. It told me that it needed Play (something), which was not available on my device. Any ideas? I'm going to try the Send it to Kindle app and see if I can send them that way.
Amazon and Google are blood enemies. You can install Google apps on the Fire but it's a kludge and not worth the effort IMO. Dropbox is great for Fire users.
The fastest way to move a lot of files is to connect the Fire to a USB port on your computer. Your computer will see it as a USB drive. Copy the files to the Fire's Documents folder. Or if you're using an SD card on the Fire, create a folder on the SD Card ("uke music" comes to mind) and copy the files to that location.
Send to Kindle will work but 1) you have to figure it out first and 2) the USB cable is faster than wi-fi and you've got a bunch of files to move all at once. Someday in the near future you'll get a new PDF file and
then you can learn how to use Send to Kindle.
There are lots of other options, including the free program "MobileSheets Companion" that runs on your computer, sends files directly into MobileSheets, and allows you to edit the MobileSheets search fields (like artist, album, genre, etc.) That's a good option for people with a ton of individual song files and a burning desire to have everything cross-indexed.
If I were you, I'd start with the USB cable. There will be plenty of time to complicate things later ;-)