We will visit Paris on July for 3 days, arrive at 15:00 to Orly on monday 11 and leave on wednesday 13 morning. We don`t know if it´s worthy to buy a pass or just the 10 ticket pass. We will only visit the city centre so we don´t know about the zones.
Thanks in advance
You could manage with just 10 tickets preloaded onto a Navigo Easy card (one per person) if you don't mind wrangling luggage on the Paris Metro (as opposed to more spacious RER).
With basic Paris Metro tickets you can take Tram 7 from Orly airport to Paris (departs from Terminal 4) until its terminus where Paris Metro line 7 begins (at Villejuif-Aragon station) its journey through Paris. From there you would use a new Metro ticket to board Paris Metro and travel to your lodging. Total cost would be 3.80€ per person (1.90€ per ticket x 2).
Another option that's slightly quicker (45 min. vs 1hr for Tram 7+Metro 7) is using Orlyval train from Orly Airport (Terminal 1-2-3) to Antony station where you'd transfer to RER B train north into central Paris. This requires either buying single-use Orlyval+RER ticket (12.10€ per person) or using a Paris Visite multi-use ticket.
Finally there's the Orlybus option for 13.30€ (using an Orlybus single-use ticket) that will take you from Orly 1-2-3 to Denfert Rochereau from where you will need to transfer onto either a Metro or RER train requiring a new Metro ticket.
Once you're inside the city center you won't need to worry about zones when riding Paris Metro, which is all one zone. Paris Metro tickets would work fine on an as-needed basis.