I have posted similar question to support. It is because I found that I cannot open the pdf e-books in tablet side which purchased from entourage web site. It is no problem to open it on e-ink side.

The support answered me that the pocket edge cannot view the password protect e-book in tablet side due to DRM.
Actually, I don't know what is DRM. I Just confirmed that users able to read unprotected pdf files in tablet side only.