TomK,
You'll probably want to use a layout condition. In your layout's properties, you'll find "Expression" under "Action", where you'll be able to open up the expression editor. You can then use something like this:
IF(FILE_EXISTS("C:\myfile.pdf"), Print, Skip)
If the pdf exists, then it will Print the layout, otherwise, it will skip it. Feel free to use a variable data field for the file name if you want to. (just don't forget to add the folder path if it's not already in the field)
Hope that helps!
Regards,
Raphaël Lalonde Lefebvre