Of course since this is a web app, I don't want to create the files, just the output stream
I do not know what is so "
of course" about that.
Webservers are very good at serving files. Writing the PDF to the server's filesystem is likely to be faster than issuing a stream and waiting for any slow link in the chain to the client. Your script will terminate faster and be ready for another request, while the web-server can take all the time needed to get the file to the client.
CountZero
A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James