I'm not sure FME is the right tool for this task. If you need/want to use FME for this I suggest finding a command line tool which can merge pdf files and use the SystemCaller to run the job.

This perhaps could also be done somehow with python in FME.

Thanks for answer ,

can you please share how can i do with python in FME

Here's something I found online which should be helpful: https://stackoverflow.com/questions/3444645/merge-pdf-files

In FME you can use a PATH reader to read the file names of the PDF files as features in FME. You can then feed them into the PythonCaller with your code.