Adobe PDF, is as the name suggests, a portable document format, designed to give the viewer the same look/layout as the original.
It is a format, not designed to be studied off electronically.
It is a format designed to be printed, such that the original document is reproduced identically when printed.
It is not cost effective for the end user to print out X number of pages to cover an entire document suite.
It is a cheap means to cover the minimum requirements.
Sure if people are happy with it, let them have it, if not, there would surely be some requirement for the user to have an identical set of manuals that is found on the aircraft.
QANTAS has the option of paper or CD. (Nil cost either option)
The CD option covers nearly all manuals, however, it too is adobe PDF. It is hardly worlds best practice or latest technology. It is a pain in the arse to study from.