Prior to Catalina (10.15) (at least in 10.14), you could click on a checkbox and it'd tick it (even if the PDF wasn't a fillable PDF). You could also click near a line and Preview would identify that there was a line there and insert a text object automatically.
Am I missing something, or were these features removed?
Best Answer
I've confirmed that this feature was removed in Catalina.
For confirmation you can see on in the Preview User Guide from Apple.
If you check the Mojave (10.14) version, it says "Click a field or a line in the form. A text annotation box automatically appears.".
If you switch over to the Catalina (10.15) version, it now only says "Click a field in the form, then type your text.".