The phrase pasarlo bien means something like "to have a good time" in sentences like, "Lo pasamos muy bien anoche." What does the "lo" in this phrase refer to? Does it replace an actual noun, or is it just an indefinite "it" like in the English sentence, "It's raining outside"?
artículos determinantes - articles that indicate gender or neuter - (see RAE). It's perfectly valid to say
Pasarlo bien or
Pasarla bien in this context.
For example, if I ask you to give me a hand moving my TV from one side of my room to another; I would tell you:
"Vamos a pasarlo (TV is masculine)
a este lado" (let's move it to this side). If I want to do the same thing with my bed; I would say:
"Vamos a pasarla (bed is feminine)
a otro lado."