Primary suggestion...
A VERY large number of organisations distribute their CAD drawings in PDF format. These drawings contain layers and these layers can flow through to the PDF.
The desktop PDF viewer allows users to turn these layers on and off (very handy, paticularly where the drawing contains lots of detail) but the mobile viewers do not.
It would be very beneficial for mobile users to also have this functionality. It could be argued that with the smaller screens on mobile devices it would be even more beneficial than on the desktop.

Related suggestion 1...
The ability to select multiple layers at a time using a standard windows shift-click would be even more useful. I intend to add this suggestion as a request in for the desktop version.

Related suggestion 2...
AutoCAD has the ability to store different named views of the same drawing with specific layers turned on or off.
If these named views were able to flow through to the PDF file then these PDF drawings potentially become much more useful again.

Currently, even though drawings are created electronically and distributed electronically, largely they still end up being printed on paper. The addition of these features would definitely help people to go fully electronic.

All of these suggestions would apply to to CAD drawings of any type (Mechanical, Structural, Civil, Architectural etc)