Im new in the new version of Mono 4.2 and they implement the new PCL libraries, My question is there is a similar function like File.Exists that works on PCL, and if I mix this libraries with the full ones, there will be a problem with the compatibility of this dll in iOs. Thank you.
File System is handled differently on each operation system, so you are going to need to write platform specific code, or use pre-made cross-platform framework's plug-ins.
I suggest you to take a look at MVVMCross framework, for mobile development in general, and specifically at the File Plugin of MVVMCross.