Just out of curiosity, as I've seen a few comments also mentioning PrinceXML. Is OneDoc an API, wrapper, etc, on top of PrinceXML? Or is it a completely new rendering engine?
As of today we are building our solution on top of PrinceXML/DocRaptor which is considered "to be the best PDF generation API, giving complete control over the documents you need to create" (cf. another comment).
As we started working on this solution less than 2 months ago, building our own renderer was not an option. But once we have validated the idea, we are definitely going to work on our own renderer to have 100% control over the workflow, and also to be able to offer a better pricing model!