Hello everyone,
I have the doubt that my last post might be misunderstanding, so I would just like to clarify that by "classical design" I meant "keeping the comboboxes": they're definitely harder to make, but easier to use and understand, especially for someone who isn't so used to computers.

About the macro for opening PDF files, if it's a problem don't bother making it: I didn't actually ask it in the first place, because before asking help here I would like to try to write a new code on my own (hoping I will manage to do it!).

As always, thank you for your kindness!