An IPD file contains data regarding contacts stored on a Blackberry phone. In order to transfer these contacts to iPhone, you need to convert the file first to something compatible with iPhone. Software which does this job is called ABC Amber BlackBerry Converter. The program has the ability to convert backed-up files to a format which is more accessible. My recommendation is to convert the file to CSV format because almost any device and application are able to understand it.
Simply, load the software, click on Open IPD, and then use the export file format, click save and you will have a proper converted file compatible with most of the applications and devices.
After this operation, use iTunes, connect your device, open the CSV file and import all contacts into the phone.