It seems that the only reliable way to convert idml to pdf, i.e. print InDesign files (.idml) as PDF documents (.pdf) is to use Adobe InDesign. The overall support for InDesign formats is very low, so you won't find many programs that would be otherwise capable of idml to pdf conversion and overall, it will be hard to work with .idml files without InDesign.
In case you know anything useful about converting idml to pdf or you deal with some particular problem, please let us know in the discussion below.