The .doc format was the default Microsoft Word format from Word 97 through Word 2003. It is a proprietary binary format — OLE2 Compound Document Structure — not the XML-based .docx format introduced in Word 2007. Despite its age, .doc files are still widely used in legal, government, and older enterprise systems. Converting them to PDF preserves the content for reading and sharing without requiring Word or a compatible application.
How DOC to PDF conversion works without Microsoft Office
LibreOffice has supported the .doc format since its earliest versions. It implements the Microsoft Office Binary File Format specification, which Microsoft has published openly since 2008. For most .doc files created between 1997 and 2007, LibreOffice produces accurate output: headings, paragraphs, tables, lists, inline images, and basic text formatting are preserved.
LibreOffice is a separate layout engine from Microsoft Word. While it handles the same file format, it applies its own rendering rules. The result is visually close to what Word produces but not guaranteed to be pixel-identical, particularly for documents that use complex table layouts, embedded OLE objects (linked charts from Excel, for example), or Word-specific drawing features.
Using Filum's DOC to PDF tool
Filum's DOC to PDF tool sends the file to a LibreOffice Writer server, which opens it and exports it as PDF. The tool requires an upload — LibreOffice runs on a remote server, not in the browser. The file is sent over an encrypted connection, converted, and immediately deleted. No account is required, and the file is never stored permanently.
If the source file was created in Word 97-2003, conversion takes 5–20 seconds depending on the file's size and complexity. Larger documents with many embedded objects may take longer.
When .doc formatting doesn't convert cleanly
Several .doc features have imperfect LibreOffice equivalents. OLE-embedded objects (Excel charts embedded in Word, for example) may be rendered as static images or may not appear at all. Documents that use complex multi-column layouts with floating frames can have elements displaced. Tracked changes and revision marks are resolved to their accepted state in the output PDF — the revision history is not preserved.
If formatting accuracy is critical and you have access to Microsoft Word, exporting directly from Word to PDF will produce the closest possible output. If you only have the .doc file and need a PDF quickly, LibreOffice's output is accurate for the vast majority of real-world documents.
The difference between .doc and .docx
.doc is the legacy binary format. .docx is the Office Open XML format introduced in Word 2007 and standardised as ECMA-376 / ISO 29500. Most modern Word files are .docx. LibreOffice handles both, but .docx conversion is generally more accurate because it is an open XML format with a complete public specification. If you can save the file as .docx from the application that created it, use the Word to PDF tool instead.