From
http://filext.com/file-extension/PK
Adobe Systems Incorporated acquired the technology assets of Syntrillium Software in May 2003. On August 18th, 2003 Adobe released a rebranded version of Cool Edit Pro 2.1 as Adobe Audition software. Audition/Cool Edit creates .PK files which are known as Peak Files. Peak files store data associated with the graphical representation of audio data (i.e., Waveform diagram). Audition/Cool Edit has the option to disable the creation of these files, however this may slow loading, saving and re-drawing of graphical data. These files can safely be deleted and will not effect the original audio data in any way.