A free option is Project Libre, which is open source and it is available for both Windows and Mac OS. It has good compatibility with MS Project files, save for some features, but I think it will most likely meet your needs. Google it, and you will find its download page in no time.
If you need a more robust tool and have money to spend, there is the paid option of OmniPlan, which can be found in the Mac App Store and has a version for iOS as well. It is pricey, though - about US$ 200 per license (one-time) - and it is proprietary.
Hope that helps.