This is a very simple process, just like most things on a Mac.
From ANY program, choose the Print option from the File menu. Wait for the print dialog box to pop up and select the PDF button in the lower left hand corner.
From the pop up menu choose "Save As PDF" and you will be prompted for file name and location information. It will save your file by default with the same file name with a pdf extension.
You can watch a short video of this process here.