Referring to a form, how would you say in idiomatic Spanish of Spain "I've left it on your desk for you to sign"? Lo he dejado en tu mesa para firmar [?]
Lo he dejado en tu mesa para que firmes [?]
Or something else?
I don't think there's a significant difference between Latin American and Peninsular Spanish in this context.
I would say something like:
Lo he dejado en tu escritorio para que lo firmes.
However, I like your second try.
Your first one is kind of ambiguous, but it can be perfectly understood in that context. I mean, the one you're talking to will understand to whom you're asking to sign.
(By the way, is this last sentence grammatically correct? : the one you're talking to will understand to whom you're asking to sign)