One of the problems with the Cisco Packet Tracer is the incompatibility between versions. Packet Tracer uses three file extensions; PKT, PKA and PKZ. The PKT files are generated when using the program and saving a file while the PKA are redistributable and new activities. PKZ, only available when using Packet Tracer 5.2, are used when packaging multiple files to create custom activities with graphics and icons of their own.
While the icons are similar between versions four and five files, PKT and PKA, when using Packet Tracer, are in fact different. Version 4 files are compatible with all later versions, while version 5.1 files are not compatible with Packet Tracer 4, but with the new version 5.2 and files from the version 5.2 (the latest version at the time of writing this article) do not have backward compatibility.
Another detail is that if you modify and save a file made in a later version, this is converted to the later version. Consider this when performing exercises on a computer that is not yours, because if you have a later version, then they will not be able to open the file. There are currently no solutions available for this, apart from always ensuring that Packet Tracer is updated to the latest version available.