Auto page size based on content size · Issue #1627 · wkhtmltopdf/wkhtmltopdf · GitHub

One aspect of unlimited page length is that, while pages are built from the top down, the coordinate system runs from the bottom up. Thus, the y-coordinate of the top of the page needs to be known when you start. It's too bad that Adobe chose to do this (almost every other graphics system has y=0 at the top). That's no problem for a fixed media size, but trying to build a page of unknown media length means that sooner or later you will go negative on your coordinates. You would need to make a second pass to update all y-coordinates to have y=0 at the bottom of the long page (as well as reset the MediaBox). The alternatives include making a regular first pass (normal media size) and determining the total length at the end, then making a second pass with the extended media size; or making a wkHTMLtoImage pass first to determine media size; or somehow gluing together regular pages into one; or simply oversizing the media and cropping the result afterwards. You might also be able to use a flipped coordinate system (y=0 at the top) and either make a second pass through to update/correct it, or use the cm operator to flip the coordinate system.

Don't forget that PDF officially limits you to 14400 points, unless you use a UserUnits scale factor. I think a lot of readers are lax in enforcing that rule, but you never know when you'll run into one that is strict about it.