In order to ensure your digital pattern is printing at the correct size, always print just the PDF page with the test square before printing the entire pattern. The test square should measure 4 inches on a side (10.2 cm).
By default, many applications will scale down the pages in your digital pattern PDF when printing. Before you print, check your print setup options and make sure that scaling is set to “100%” or “None”, depending on how the option is worded.
You may also sometimes need to set this in your software preferences/settings as well. Adobe Reader on the Mac, for example, often needs a change in the application preferences in addition to the scaling setting on the print dialog.
The following screenshots should help you find this scaling setting on Mac OS X and Windows, respectively.
Note for Windows 8 users: Windows 8 has limited ability to work with PDFs without a third-party application. If you do not see a print settings dialog similar to the above when printing your PDFs, you will likely need a separate program such as the free Foxit Reader.