Preview can export content as PDF, or by selecting single or multiple thumbnails, export those as PDF. I have already done this on Mojave, so I know that works. The functionality of your Samsung Xpress SL-C480 is dependent on Samsung (now HP) keeping the drivers current for Mojave (which they haven't), or your checking these drivers before an upgrade to Mojave to determine their potential limitations.
What I see is that the basic drivers for this multi-function device were last updated in Sept 2017 (just before High Sierra was released), and that there was a firmware update in Jan 2018. The Samsung Easy Document Creator, and Samsung Easy Color Manager software were updated in Aug 2018.
Essentially, what you are faced with now is SL-C480 installed drivers that may not be compatible with Mojave. Check this HP page to ensure that you have the latest available drivers, Reset your Print System, and re-add the device. If this does not solve your issues, then contact HP about their future Mojave commitment for this MFP device.