maybe not Ruller tool... something like scale bar at the top of the page (see ms word)
Eduardo Jiménez commented
I would love to have a built in ruler on the app. This would work great for physics textbook problems and math problems in general.
Two tool bar options.
1) Set measurement (To a line, per page)
2) Measure between two points (Via Set Measurement tool bar option)
This is an idea I had while using your app for viewing blueprints on a construction site.
A PDF ruler to measure things in document, because sometimes is necessary take values from pictures.