The PBT format is associated with an archiving system called MARS (Micro-Dynamics Archival and Retrieval System). In case you found or received a PBT file, it is probably old and rare, so opening it might require conversion.
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 PBT files as once.
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