Anyone who has worked with PDF files might question the accuracy of a printable ruler, considering how flexible the print settings are in Acrobat Reader. Certainly older versions of Reader were a little off in reproducing exact dimensions, even with the print settings left alone. Not to mention that the default settings seemed to change from one version to the next, making it nearly impossible to design a PDF that behaved the same on all computers.
Recent versions of Reader seem to have the problem beat. After making a few tests, I was surprised to find that Reader 8 provided me an extremely accurate printable ruler, at least accurate enough that I couldn't see any difference between my 12 inch paper ruler and the metal ruler from my desk. I think it is still important, though, to check your printer settings before clicking the print button. If anything gets changed from the basic default (this can happen), your ruler will not be accurate. Here's what to look for.
In Acrobat Reader, clicking the printer icon in the upper left corner will launch the print dialogue box. Look for the section called Page Handling and see the drop down menu for Page Scaling. You have several choices here, so look over the following descriptions below to find the best
Page Scaling: None
This is the default setting for Acrobat 8 - and the setting you must use if you want your printable ruler to be accurate.
Page Scaling: Fit to Printable Area
This is a handy feature if you want to print pages that are larger than usual (like legal) but your printer is loaded up with standard letter-size paper. No problem. Acrobat will squeeze everything down to fit.
Page Scaling: Shrink to Printable Area
Very similar to the previous setting (fit to printable area), but rather than automatically shrinking all the pages in the document, Reader will go through each page to see it does or doesn't need shrinking to fit on your letter size paper. Also not a good setting for a printable ruler.
See my favorite Printable Rulers