In general when travelling in and out Italy you should check to have with you a valid passport and your residence permit (or the post office receipt if you are waiting for the first issue).
You will be asked to show them to the police officers at the Italian borders.
In general Non EU citizen can stay outside of Italy for a continuous period provided that it does not exceed half the duration of the residence permit.
The EU calculator tool works out when to enter again the Schengen area and how many days you are allowed to stay there.