We use a robot file when we want to schedule a script that isn't fully server schedule compatible or which would require jumping through a lot of extra hoops we don't care to take on...
Define a filemaker file with nothing but the needed password settings and a two line script:
Perform Script [// specify the script in your hosted file that will do the save as PDF task]
Exit Application // or Close File if you want to leave filemaker open
Use file options to set the file to open the file with a password that has sufficient privileges to run the script (can be a very limited password if the script in the hosted file is set to run with full access permissions.) and also to run the above one line script.
Use Windows Task Scheduler to "run" the above filemaker file at the desired time.
Note that if the script being performed get's trapped in an infinite loop, the only way I know to terminate the looping script is to close and reopen the file via the server admin application.
I was doing exactly like your idea to use Windows Task Scheduler. I keep the machine that runs this schedule script on, and it can save the A/R report as a PDF at the end of each month. It took me a while to firgue out the last day for February for leap year.