There are many awesome sellers of PDF hand embroidery patterns on Etsy, and Jenny Hart of Sublime Stitching recently started selling PDFs of her patterns as well. How do yo use a PDF embroidery pattern once you purchase it, though?
1) You need a program that can open PDFs. The most common is Adobe Acrobat Reader, available for free here: http://get.adobe.com/reader/ Mozilla offers the FoxIt PDF Reader for free on their website as well: http://www.foxitsoftware.com/
2) Next you'll want to print the pattern. If the pattern is multiple pages, I recommend printing just the page you need. Most printers will allow you to choose advanced printing options and "scale" the size of the printout as well, so if you want to print a smaller/larger version of the pattern you can. The exact directions for changing the size will vary with different printers. Some people put the pattern up on their computer screen and trace directly off the screen, but I don't recommend that. It will damage your computer screen.
3) Now, you need to choose a method to get the pattern onto your fabric. You can use a transfer marker or pencil, transfer paper, or even a lightbox/window and tracing method. Read Amy Bindel's "How To" at Sublime Stitching's website: http://www.sublimestitching.com/transfertechniques.html
4) Once you've transfered the pattern to your fabric, you're ready to stitch!