PD is a specialized file format used by some medical devices which perform tomography or CAT scan. It is considered rare and it's probably difficult to open using regular image viewers. For best results, PD files can be converted to more accessible formats.
The quick and simple way to handle your files is to get a quality piece of software, such as reaConverter. This software is extremely efficient in managing a wide range of batch conversions. As you will soon realize, reaConverter will help you avoid spending countless hours trying to figure out how to convert multiple PD files as once.
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