Media player is not able to open .pdf files by design, it is for media, like movies and music. You need Acrobat reader or another .pdf program to open them.
Thx. for your advice. It's only certain pdf files, that I am unable to open.They all have either pictures or short videos
in it, that triggers the Windows Media Player warning, that selected file has a pdf extension, that is unknown or not recognized by the player. If I transfer the file to a XP driven computer, in most cases I am able to open the file. It's
just Win 7 that creates problems.