Any document or picture can be easily exported / printed as PDF document simply by using virtual PDF printer or some kind of PDF export function that does the same thing. As for RIFF format, this one is typically used in Corel software (original Corel Painter), so using CorelDRAW might be actually the best thing how to achieve riff to pdf conversion. But any graphics viewer/editor that supports RIFF format should do.
Updated: August 28, 2023