How can I create plaintext files with Markdown syntax and convert them to PDF format?

I am very much a novice at this, so please bear with me. I'm intrigued by the idea of using Markdown. I want to be able to write my stuff in a plaintext editor on OS X and iOS and save it in Dropbox in .txt format using a Markdown syntax. I don't want to save my files in markdown format but as plain text. I need support for footnotes, so the Multimarkdown syntax seems like an obvious option. What tools can I use on OS X and iOS to convert my .txt files, marked up with a Markdown syntax, to .pdf format? Is this even possible? Thanks for reading.