OPF stands for Open Packaging Format. The Open Packaging Format is a standard that is a part of the EPUB eBook specification. The OPF files store information for an associated eBook including the title of the eBook, the eBook language, creator, reading order and book ID. The OPF file is included in the EPUB archive to specify the metadata of the associated eBook. The OPF files are saved in XML format.
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If you have additional information about the OPF file format or software that uses files with the OPF suffix, please do get in touch - we would love hearing from you.