What is an OPF file?
An .OPF file is an Open Packaging Format document. It acts as the core manifest for EPUB e-books. The .OPF file stores important metadata, such as the title, author, language, and reading order. It also lists all the separate files—like text, images, and stylesheets—that make up the complete e-book. These files are typically developed by the International Digital Publishing Forum and use the standard XML data structure.
While most .OPF files belong to e-books, the extension has multiple possible meanings. It is also used as a FlipViewer FlipBook File or FlipAlbum File created by E-Book Systems. Other industrial applications use it as a KEPServerEX or OPC server project file for Kepware, or as an OpenPlant project file by Bentley Systems.
How to open OPF files?
If the .OPF file is part of an EPUB, you can open it with e-book management software like Calibre. Because standard .OPF files are written in plain text XML, you can also open and inspect them using basic text editors like Notepad++ or Visual Studio Code. If the file belongs to an industrial software or is a FlipViewer document, you will need the specific native software to open it correctly.
Since an .OPF file can have multiple meanings, viewer.online/opf analyzes .OPF files to identify their exact format and creator software, shows which programs can open the file, and usually previews it. If you are unsure what your file contains, we recommend using viewer.online/opf to identify the actual format of the user's file.
Software and tools
To manage and read standard e-book packages, Calibre is the primary tool. If you need to change your e-book format for a specific device, you can convert the entire package. We recommend file conversion to sensible target formats on convert.guru, where you can change e-books into PDF, EPUB, or TXT files.
Summary
Identifying and viewing an unknown file can be confusing. Because .OPF files use the known XML format, viewer.online/opf can safely open and display them online, eliminating compatibility problems.