For weeks I was not able to download "borrowed" library Adobe PDF ebooks to my Sony Reader from Adobe Digital Editions. I kept getting a message that my PRS505 was not authorized for DRM material. Adobe was no help. A technician at my public library had more direct contact with Adobe's support personnel, and even he could not get adequate assistance. FINALLY, he got the correct solution from Adobe. I hope it helps you, since it corrected my problem.
Connect your PRS-505 to the PC, open Adobe Digital Editions, and press CTRL+Shift+E on your keyboard. This will deauthorize the device.
Then exit ADE and re-connect. You should see the message that your Reader needs to be authorized AND see the link to properly accomplish this. It worked for me. Bon Chance!