Files with .PST extension represent Outlook Personal Storage Files (also called Personal Storage Table) that store variety of user information. User information is stored in folders of different types that include emails, calendar items, notes, contacts, and several other file formats. PST files are used for archiving emailing data offline that can be later loaded and viewed in various applications.
Files with .EPUB extension are an e-book file format that provide a standard digital publication format for publishers and consumers. The format has been so common by now that it is supported by many e-readers and software applications. For example, on Mac OS, the pre-installed Books software provides the support for opening such files. In addition, there are a lot of compatible software available for smartphones, tablets and computers.
You can also convert PST into many other file formats. Please see the complete list below.