When I use the heading tags (# ##, etc) using markdown, they are always converted to numbered sections during pdf-latex conversion.
How to indicate unnumbered headin
You can write an unnumbered heading by appending {-} or {.unnumbered} to the Markdown cell, e.g.:
{-}
{.unnumbered}
Heading {-}
The output is a heading without a number, like so:
This is documented in Pandoc's Markdown which is an extended and slightly revised version of the original Markdown syntax.