You can download Secure Adobe PDF and ePub to your Mac using Adobe Digital Editions. They will work "as is" on your PRS-505 if you register it to your Adobe ID from a Windows PC (a one time operation). The DRM can instead be circumvented, but you may need to use a Windows PC (one time) to get a valid key.
If you really want to register nothing, then eReader format is your best bet for Secure ebooks, but it requires a credit card number. Its DRM is circumvented using ereader2html.py, or for best results see
ereader2ereader in two steps.
Another option is Secure MOBI, but it requires a PID from a supported device (e.g. a Windows PC). You don't ever need to use the device once you have the PID. The ebooks can be downloaded using any web browser. Use mobidedrm to strip the DRM.
With eReader or MOBI, use Calibre on the DRM-free ebooks to convert to a PRS-505 format (LRF or ePub).
Don't forget that circumventing DRM is thought by some to be illegal in the US and several other countries.