How can I create plaintext files with Markdown syntax and convert them to PDF format?
I use Marked to process text files to pdf routinely.

No matter what editor you use, Marked will open and re-save a PDF generated from the plain text input by opening and re-saving the file. 

It also saves to HTML or RDF output from markdown.

In reality, any program that renders markdown could print to PDF using OS X core services (File -> Print -> PDF -> Save as PDF), but I much prefer having Marked for it's multitude of handy and flexible processing options and it's ability to live preview markdown files as I edit them in another editor:


Printing: TOC, suppressing link underline colors, better page breaks
Multiple styles of markdown rendering
Support for custom processors, all manner of smart (and dumb) typography
overall clean, fast, robust, elegant software for a very reasonable price