Apple won't ever offer a downgrade. You have to go looking for it and usually a downgrade involves a nuke/pave approach where you reformat and repartition the drive completely to get rid of the more modern version and install from scratch. If you have Catalina on it now, just leave it there. All of the Adobe products should work (unless they are very, very old versions you are attempting to install). To get back to Mojave, you would have to totally erase the hard drive as Catalina has made significant changes to it that will not work with Mojave. I'm running Catalina and it's just fine. All you need to avoid are 32-bit applications as Catalina won't run them at all.
Just a comment on a previous post, just to be clear:
Bootcamp is not downloaded as an add-on, it is a component of macOS that allows you to install Windows in a separate partition and dual boot. To do that you need a license for Windows. You run bootcamp from macOS to create the partition for Windows, then install Windows in that partition and then install tools for macOS to be able to offer the option to boot the OS you want. If the previous owner had Windows in Bootcamp, and if the installer saw the partition, then the hard drive is still divided and you are not getting full access to it. So going back to Mojave and reformatting the ENTIRE drive will get that space back for you as a side benefit.
gotcha. that sounds a little bit more complicated for someone like me to get involved with. i think i'll just stick with catalina as you said, even though it seems like mojave is awesome and i haven't seen many people brag too much about catalina... idk. as long as adobe products work well then i'm golden.
the bootcamp thing confused me, as it must be something the previous owner did. it would be nice to get that extra 50GB of space back that bootcamp is using, but might be not worth the effort involved (for someone relatively inexperienced like me at least).
thanks for your input.
Apple's link to download Mojave from the Mac App Store: LINK
thank you my friend