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She's probably already arrived in Paris by now.
Literal
She [topic-は] around-this-time already Paris [at-に] has-arrived [probably-だろう].
今頃 ('around this time, by now') conjures the moment — it places the speaker mentally at the present and asks where the absent person should be in their journey. Combined with もう ('already'), it sharpens the inference: 'enough time has passed.' The resulting-state ~ている on 着く ('arrive') turns the action into a state ('is in arrived condition'), and だろう puts a probability frame around the whole inference. The chain 今頃もう…ているだろう is a stock template for absent-person speculation.