I recently read this article by Jeremy Clarke on the Knack blog - http://www.knackhq.com/links/blogs/dynamic-pdfs-and-documents-with-webmerge.html
This feature would be a very valuable addition to Knack and I think more could be done to improve the integration further i.e. removing the need to use jQuery and instead adding a button generator to enable WebMerge buttons to be created instantly from the Knack interface and added directly to forms.
It would also be useful if the documents generated by WebMerge could be automatically stored in a "files" object along with a version number to ensure a permanent record of the documents is retained (for audit/legal purposes).
I also think WebMerge is expensive and would really like to see a partnership between Knack and WebMerge. Perhaps WebMerge could offer their starter plan to Knack customers for free with the potential of upgrading to their more expensive plans at preferential rates?
Otherwise the functionality offered by WebMerge should be directly integrated into Knack.