Files with the extension APXL contain the settings of a presentation of the presentation program Apple Keynote. The file contains plain text in an XML structure. It can be opened with any standard text editor. An APXL file defines the properties of a presentation, from the look to the transitions between the slides.
The APXL file is usually part of an archive with the extension key that contains the entire presentation.
Keynote is also available as part of the Apple iWork office suite.