Now, I'm not sure about the specifics of your situation.
But, if you are generating some kind of report.. that is.. maybe one person requests pdf output once a day or so.. You could do things like..

Autogenerate the pdf output once a day (this is imagining you already know what the output is going to/should be )

Maybe instead of letting them download it.. When the request the pdf.. it can be sent via email to them instead. You would have to keep track of it- that the user making a request does not already have a request in progress.

Another thing I would look into.. (possibly.. again.. if it's worth the human time (yours)!) is named pipes (fifo), and maybe in conjunction with daemonizing a process (In essence, the request is put in a queue).