Many split archives (especially from forums) are password-protected. Check the source where you found the link for the key.
The most common reason for errors is a missing piece. You cannot open .7z.001 unless you have every other numbered part in the set (e.g., PAd.7z.002 , PAd.7z.003 , etc.) saved in the . 2. Verify File Names PAd.7z.001
This means one of the parts was corrupted during download. You may need to use a "Repair" tool if a .rev file was provided, or simply re-download. or simply re-download.