LinuxWhat is the OPF file format?
File with the .opf extension stores digital publication metadata in plain-text XML format. The .opf format conforms to the Open Packaging Format standard, which is the core package document of the EPUB ebook specification - now maintained by W3C (formerly by IDPF). .opf files store such information as:
- Title,
- Author,
- Language,
- Publication ID - usually an ISBN identifier,
- Reading order (spine),
- Navigation method,
- Formatting directives,
- Pointers to optional elements,
- Descriptions and references to all parts of the publication (manifest).
.opf files are part of a larger file archive that constitutes an .epub file. In EPUB 2, an .opf file required a related .ncx file - which contains the table of contents - to be present in the package; EPUB 3 replaced that requirement with an XHTML Navigation Document (nav.xhtml), though .ncx may still be included for backward compatibility. The EPUB 3 package level also supports backward compatibility, though a reader that implements only EPUB 3 may ignore legacy data from older EPUB 2 files. The package additionally contains CSS, SVG, and other media files, alongside a META-INF/container.xml file that locates the package document. An .opf file can be accessed by unzipping an .epub archive. Its filename is typically content.opf, but it can be anything - META-INF/container.xml holds the authoritative reference to it within the package.
Security & safety
RISK: LOWAn OPF is plain XML metadata - no executable content, effectively no risk to open in a text editor or e-book editor. The only caveat: it can reference external resources, but readers don't auto-fetch them. The bigger issue is user confusion (expecting a readable book), not security.
Format details
in a nutshell- FlexiSIGN / sign-making project file (.opf) - Some signage/cutting software (FlexiSIGN-PRO lineage) saves projects as .opf - unrelated binary format.
- Generic 'options/preferences' files - A handful of apps name a settings file .opf; contents are app-specific.
Programs that open OPF files
Windows3 apps
Calibre
Open-source To edit: Calibre's 'Edit book' opens the whole EPUB including its OPF. To read a metadata.opf, you don't - it just feeds Calibre's library metadata. 
Sigil
Open-source EPUB editor; open the EPUB and Sigil exposes the OPF (manifest/spine/metadata) for editing. 
Any text editor (Notepad++ / VS Code)
Open-source Open the .opf to read/edit the raw XML metadata and manifest - it's plain text. 
macOS2 apps
Technical details
deep spec| File structure | Plain-text XML; no binary header or magic bytes |
| MIME type | `application/oebps-package+xml` |
| XML root element | `<package>` in namespace `http://www.idpf.org/2007/opf` |
| Required child elements | `<metadata>`, `<manifest>`, `<spine>` |
| Default filename | `content.opf`; also commonly `package.opf` |
| Located via | `META-INF/container.xml` inside the `.epub` ZIP container |
| Spine element | Orders `<itemref>` entries that define the default reading order of content documents |
| Manifest element | Lists every publication resource - HTML, CSS, images, fonts, and media files |
| Metadata vocabulary | Dublin Core elements (`dc:title`, `dc:creator`, `dc:language`, etc.) plus EPUB-specific `<meta>` properties |
| Navigation (EPUB 2) | References a `.ncx` file via the `<spine toc="…">` attribute |
| Navigation (EPUB 3) | References an XHTML `nav.xhtml` Navigation Document via manifest item `properties="nav"` |
| Optional EPUB 2 element | `<guide>` - provides legacy navigation landmarks (deprecated in EPUB 3) |
| Optional EPUB 3 elements | `<collection>` for grouping related resources; `<bindings>` (deprecated in EPUB 3.3) |
| Character encoding | UTF-8 required; BOM not recommended by the specification |
| Accessibility metadata (EPUB 3.3) | Supports `schema:accessibilitySummary`, `schema:accessMode`, and conformance claims inside `<metadata>` |
| Developer / maintainer | W3C (transferred from IDPF in 2017) |
| Released | 2007 (EPUB 2.0, IDPF); current EPUB 3.3 (W3C Recommendation, May 2023) |
| Open standard | Yes · royalty-free |
| Specification | www.w3.org |
OPF conversions
Community Q&A
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