I made this recording back in 2009 to show how you could use Adobe Acrobat Pro to scale a floor plan, and ultimately find the square footage of a given area.
More often than not, I'm faced with figuring out the square footage of a building layout with no scale indicated on the drawing. Adobe Acrobat Pro has a measuring tool built into the application. You can take any image file, find the scale ratio of the drawing, set the scale, then calculate the square footage of the floor area.
The measurement tool line in newer versions of Adobe Acrobat Pro is a red line, instead of black, and it is easier to see the perimeter area you're drawing. Other than that, the video should help you get more out of an application you already have, but maybe weren't using for this task.