HTML to PDF on Google AppEngine
I ran into this same problem a year back and concluded that this is currently not possible in App Engine, at least with a good quality conversion. (Someone please point out if things have changed)
xhtml2pdf - I was able to successfully run it in standard App Engine but not at all happy with the conversion quality.
PDFkit - Ran into a similar problem and came up with a different solution. Hosted PDFkit on a Compute Engine Instance and exposed an endpoint wherein a POST request with the HTML file will return the converted PDF as a response. This gave me the best/expected results in terms of quality/speed of processing. 
It did incur some extra charges but I was able to utilize the instance for something else too ;). I chose the least possible configuration initially since I was not storing anything on the Compute Engine Instance.
We're currently trying to convert html files to PDF on AppEngine using Python. The HTML files are from a third-party vendor so we have no control over their format. Both the Flexible and Standard environments are options, but every path we go down we seem to hit a roadblock:

PDFkit requires a wkhtml2pdf install, no PIP package available, however converts perfectly offline
xhtml2pdf / PISA - works even on GAE Standard but doesn't support many features such as float and badly formatted HTML
WeasyPrint - C dependencies in theory would run on the Flexible environment but no pip packages available for dependencies including Cairo and Pango

Has anyone got a robust solution running on AppEngine with any of the above? Or with other libraries I am missing?