Welcome to MobileRead
I have the 505 and love it. I hope you find time for leisure reading as this site has a wide variety of beautifully formatted ebooks by some of our dedicated members. Don't forget to checkout the Deals and Freebies (we have many authors who are members and also offer some of their books).
I download pdfs as a format of last choice (even though they do work on the Sony) and prefer the lrf and epub. Next, lrx (Sony DRM version of lrf), then ADE (Adobe Digital Editions).
The Wiki is a good place to check for additional information or just jump in and ask. The members on this site are very willing to share their knowledge or point you in the right direction.
As to your question about the password protected pdf... This is from the FAQ on their site.... (also for those of us who are reading your post and want more information about the question

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1. What does an eBook look like? Can you give me an example?
An Apress eBook is essentially a password-protected PDF, which you can open using the free Adobe Reader. You can download an example on the right if you are unsure if your PDF software can open it. If you have an older version of Adobe Reader (<4.0), please consider upgrading your software.
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3. What is the password for my eBook? Can I remove it?
The password for your eBook is your account e-mail address, case sensitive. We rely on PDF's password encryption to protect our eBooks from digital piracy. Until a better approach comes up, the eBook will require a password to open.
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5. Will an eBook work with other handheld devices?
Currently our eBooks are only supported on a PC/Mac, however they may work on other portable devices but are not guaranteed. The only portable device that is guaranteed to work with our books is the Kindle, however the eBooks for the Kindle needs to be purchased through Amazon.com. (Purchasing an eBook through Apress.com and loading it onto the Kindle will not work.)
It seems that this site protects their work by making you input the password in order to open and read it. As to the Kindle version, Amazon probably adds their DRM so it will open on the Kindle.
I hope we have helped.