As Charles stated there are two options, I would love to use both! So you could make straight lines with a virtual tool on a specific place because of side rulers, like MS did.
Maybe for the tool part you could implement other types as well, like the protractor?
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.