I dont think that we have native support for TIFF files in PRPC, to my knowledge, and there is no OOTB utility available for doing PDF-to-TIFF conversions and vice versa. Seems like you would need to locate a 3rd-party tool and add it to the PRPC classpath, then write a utility function that can be applied where it is needed.
Santanu is correct. Pega has no native capabilities for converting TIFF files to PDF. I believe the only support for TIFF is the Pega Image Viewer, but that does not have a "Save As" feature for converting one format to another. Best bet is the one Santanu describes, which is to extend Pega capabilities using a 3rd-party lib.