Just as a bit of background, this is a feature that we have considered in the past. We have not implemented it in Document! X primarily because the nature of output produced for Sdk style reference documentation does not lend itself well at all to a printable medium. The familiar structure of documentation is so based on linkage, dynamic filtering and other features that simply don't work in a printed medium. Also, the sheer scale of output particularly for .NET documentation would make a printable document unusable at best and an eco-disaster in felled trees at worst!
All that said, if anyone has a different view on how we could make a printable output work well in Document! X from a design point of view we would love to hear your ideas and will certainly consider this as a feature for the future if we can square up a good design.