Probably the easiest way to transfer files is to upload your PDF files to a cloud storage service (e.g. Acrobat.com, Dropbox, Google Drive) from your desktop computer and download the files to your Android tablet.
Adobe Reader for Android has integrated support for Acrobat.com.
- Open a web browser.
- Go to https://files.acrobat.com
- Create a free Acrobat.com account.
- Sign in and upload your PDF files to Acrobat.com from your desktop computer.
- Launch Adobe Reader for Android on your tablet.
- Go to the home screen.
- Tap Acrobat.com.
- Sign in.
You should be able to access all of the files (that you've uploaded from your desktop computer) in Adobe Reader on your tablet.