.APNX File Extension
File TypeAmazon Page Number Index File
What is an APNX file?
Pagination file used by Amazon Kindle devices starting with the 3.1 firmware version; maps page numbers of the digital book to the page numbers of the original print book; allows Kindle readers to see page numbers; saved on the device and not meant to be opened or edited manually.
APNX files are often made available with Amazon eBooks (.AZW file).