Format called PEF was created by company named Pentax. Its whole name is Pentax Electronic File and it is optimized for cameras by Pentax. This company is originally from Japan, but today is ongoing its second acquisition into Ricoh company. First was done into company called Hoya. PEF file format is very favorable because of its creation of not edited and not compressed photos. Its main disadvantage is enormous size of files before editing. That´s why it is not used immediately, but it is better to modify them, reformat them for example into .jpg, so you will prevent data loss and you will use maximal potential of photography.
To open files saved in this file format you can use application created by Adobe Systems and it is known as Adobe Photoshop. Unfortunately, it is not available for free. Adobe Photoshop has only its 30 day trial version. However there is also program Able Rawer, which is available for free and it also can open files saved in this file format.