I bought the MB-339 today on Orbx Central and noticed the blank pages too. I posted a comment about this on the developer's blog under the post for pre-release beta testing of the MB-339. This was the only reference to the product I could see on his site. I thought there might be a legacy FSX version of the product for which there might be a manual but I couldn't find any reference to it. He apparently had a freeware addon of the earlier MB-226 years ago (no manual I could see). I was able to install and fly the aircraft successfully in MSFS but I would like to know more details. At least the first part of the PDF identifies the front panel instruments.
-Bruce Irving