For long-term storage, PDF is the better choice in almost every case — and for documents that will be archived for decades, PDF/A is specifically designed for the job. DOCX is a better working format for documents you still need to edit, but for anything you're putting away and may not look at for years, the structural differences between the formats matter significantly.
The Core Problem With DOCX for Long-Term Storage
DOCX files depend on external resources to render correctly. Fonts referenced in the document may not be installed on whatever computer opens the file in the future. The rendering behavior of Word versions changes across releases — a document formatted precisely in Word 2010 may look noticeably different in Word 2030. Features deprecated in future versions can cause content to render incorrectly or throw errors.
Consider a contract stored as DOCX for 15 years. When it's eventually needed, the fonts used in 2025 may not be installed on any available system. Word's rendering of those styles may have changed. The document that opens might look different from the one that was signed. For a document that needs to be an accurate record of what was agreed, that's a problem.
Why PDF Holds Up Better Over Time
PDF embeds everything needed to render the document inside the file itself. Fonts are embedded, color profiles are embedded, the page layout is fixed. A PDF created in 2005 opens today looking exactly as it did then, on different software, on different operating systems, on different hardware. The format was explicitly designed for this — fixed, self-contained, portable.
PDF is also an ISO standard (ISO 32000), which means its specification is publicly documented and maintained independently of any single software vendor. Any company can implement a PDF reader, now or in the future, by following the published standard. DOCX is also an ISO standard (OOXML), but its complexity and Microsoft-specific extensions mean that independent implementations often diverge from how Word renders files.
PDF/A: The Archival-Specific Variant
For documents meant to last decades, PDF/A goes further than standard PDF. It prohibits features that create external dependencies or may not be supported by future software: encryption, JavaScript, audio and video embedding, external color references. What remains is a fully self-contained document that can be rendered by any compliant PDF/A viewer without any external resources.
PDF/A is the standard mandated by government archives, court systems, and regulated industries precisely because of this future-proofing. If you're storing documents you may need to produce in legal proceedings or compliance audits years from now, PDF/A is the format that eliminates rendering uncertainty.
When to Keep DOCX Alongside PDF
For documents you might need to edit in the future — templates, form letters, policies that get updated periodically — keeping the DOCX source alongside the PDF archive makes sense. The PDF is the fixed record of what the document said at a specific point in time. The DOCX is the working version you update when things change. Both serve a purpose; they're not alternatives to each other in this context.
For documents that are complete and won't be revised — a signed contract, a tax return, a legal notice, a completed report — there's no reason to keep the DOCX once the PDF is created. The DOCX is a working file; the PDF is the deliverable. Store the deliverable.
Practical Recommendations by Document Type
- Contracts and signed agreements: PDF (or PDF/A for anything with significant legal weight)
- Tax returns and financial records: PDF/A — these are exactly the documents that need to survive intact for 7-10 years
- Policies and templates that get updated: DOCX as working file, PDF export for each published version
- Reports and deliverables: PDF Tools to create and archive; DOCX only if someone explicitly needs the editable version
The principle is simple: use DOCX while you're working, use PDF when you're done. For anything going into long-term storage, PDF's self-contained nature and standardized specification make it the more reliable choice over any timeline longer than a few years.