Can an RTF in a container field be converted to PDF on-the-fly using PHP API?
The FM API for PHP is a small set of classes/methods/functions that allow easy access to send or receive data with FileMaker Server hosted databases. There would be no build-in method (in the API), however once you extract what is in a field, you have the rest of PHP to do whatever you desire with that data.
Is it text in a container field?
It's an RTF file.
You might research 'RTF to PDF with PHP' to see what others have tried.
Retrieving data ...