1) What is the password to open the Pepakura PDO file?
None of the Pepakura PDO files posted on Tektonten require a password in order to open. PDO files (papercraft templates) are created with Pepakura Designer. PDO files may be opened, viewed and printed using Pepakura Viewer. Both programs are available for download here. Pepakura Viewer is free. Pepakura Designer is not. If the creator of a papercraft template has locked a PDO file to prevent editing, then the PDO can only be opened with Pepakura Viewer. If you attempt to open a locked PDO file with Pepakura Designer, then you will get a pop-up asking for a password. So, the easiest way to avoid the password problem is to always open PDO files with Pepakura Viewer.
2) What kind of paper should I use?
I recommend using 110 lb (200 gsm) cardstock. This weight paper works well for 90% of the models posted on Tektonten. For extremely small models (like these), paper lighter than 110 lb (200 gsm) is easier to work with. For extremely large papercraft (like this), then paper heavier than 110 lb (200 gsm) will help keep the model from crumpling under its own weight.
Have a different question than these? Rather than "reinvent the wheel" I refer you to the fine websites listed below, each of which was created by a papercraft enthusiast more eloquent than myself in explaining the craft: