Password protecting a PDF is useless because there are PW removers.
There are a few scripts about that can secure your PDF but they are extremely complex. I was going to make a site similar to E-Junkie that done this on the fly but it's a rather large project that I am leaving to one side at the moment but basically here is what the script does:
Bear in mind, this is all on the fly:
1) User makes payment, once payment has been made database adds +1 to the file availability so that the file can be downloaded by the user.
2) When the user clicks download a script will write a watermark to the PDF on every page where every page will have the IP address used to pay for the item, the users name, address, paypal etc, everything will be watermarked to the PDF to prevent them from wanting to share the product.
In the end there is no real way to protect content because it can be re-written, converted to word and watermarks removed. Passwords can be removed etc.
You are better off including things in the PDF that you will benefit from, E.g. Referral links to affiliate networks and such. I have done this with a PDF that I wrote in the past, it was shared and after around 6 months I was earning $500-$1K/month in referral commissions across a few networks.
Edit: That wasn't such a bad post for my 2000th post
