Will you still be able to open these files in twenty years?

A file can survive perfectly on a disk and still become unusable. The application may disappear, a linked font may be missing, a licence server may be unavailable or the encryption key may be lost. Long-term preservation concerns the ability to understand and use the information, not only the continued existence of its bytes.

No file format guarantees twenty years of access. A practical plan reduces foreseeable dependencies and creates occasions to notice problems while recovery is still possible.

Choose formats that give a future reader several ways in

For ordinary viewing, JPEG, PNG, well-made PDF and plain UTF-8 text are strong long-term choices: their specifications are available and support is spread across many applications. A niche project file tied to one discontinued application or a licence server has a more fragile route to access.

The useful distinction is between viewing the result and continuing the work. A PDF can preserve the issued drawing; the editable drawing also needs geometry, layers, units, fonts and linked resources. Keep the native original and add an export for the next reader.

The Library of Congress includes JPEG/PNG among preferred still-image formats and identifies PDF, PDF/A and PDF/UA among its page-layout preferences. Its design guidance lists DXF as a preferred vector format and DWG as acceptable. These are useful signals of support and documentation when choosing a preservation set. Images, documents, design and CAD.

A CAD example: keep DWG, add DXF and issued PDF

For a completed floor plan, retain floor-plan-final.dwg with its external references, fonts where you can retain them, images, plot settings and application version. Export floor-plan-exchange.dxf in an agreed version and keep floor-plan-issued.pdf as the visual record of the issue. A short readme states units, coordinate reference, export version and which file is authoritative for which purpose.

ASCII DXF is useful because it is documented, can be inspected as text and is supported by a wider range of applications than Autodesk's binary DXF variant. Autodesk also exposes export precision and whether all objects or only selected objects are included. Record those choices. Autodesk: DXF save options.

Open the DXF in another suitable application. Measure a known 5,000 mm wall; verify that it remains 5,000 mm. Check one dimension, one block, non-ASCII text, layer visibility and a plotted sheet. Specialist custom objects may need an additional simplified export. Retaining the DWG protects the richer native information while the DXF and PDF provide independent routes to reuse and viewing.

For a BIM project the same pattern is native model + IFC + issued documents. Test a few required properties as well as the geometry: a model that looks right can still have lost the asset identifiers needed by operations.

A PDF example: preserve a self-contained document

For a report that must remain readable, keep real text, embedded fonts, document structure and the images needed to understand it. If your workflow supports PDF/A, validate the exported file against the selected profile. Renaming a file or selecting a preset alone does not establish that it meets that profile. Keep the source document when later revision matters.

Separate the preservation master from a smaller delivery copy. Compression can reduce storage and transfer costs substantially; retain important detail and verify the result at the intended reading size. The PDF compression example lets you compare actual downloads. For a digitally signed record, retain the original signed bytes and distribute any optimised derivative as a separate copy.

Decide what must survive

For a photograph, you may need the image, date and description. For a spreadsheet, the important content may include formulas, several worksheets and the assumptions behind a result. For a building model, geometry alone may be insufficient without properties, coordinates, linked files and an explanation of the model's purpose.

The Digital Preservation Coalition's handbook emphasises identifying significant properties and validating migrations. It also explains that a file may depend on a wider software and hardware environment. DPC: file formats and standards.

Write an acceptance statement before choosing exports: “A future reader must reproduce the issued room areas and understand the coordinate system” is more useful than “save everything as PDF”.

A worked example: a completed design project

A fictional project folder contains a native model, linked drawings, a spreadsheet of quantities and the final issued report. The project owner preserves the native files and records application versions and relevant dependencies. They also create suitable viewing and exchange exports, with their purpose and limitations noted.

The report's PDF preserves the issued presentation, but not every editable model property. A CSV export of the quantity sheet may preserve tabular values while losing formulas, formatting and other worksheets. An IFC export may support model exchange, but its content still needs checking against the intended information requirements.

A small readme records the project, dates, authoritative versions, units, coordinates, file relationships and known omissions. It contains no passwords. Another person uses this package to locate the issued report and verify a selected quantity without relying on the original author's memory.

Keep an inventory, not an unexplained pile

For each important collection, record its owner, location, approximate size, formats, sensitive content, retention decision and required software or keys. Distinguish material that must remain editable from material that only needs reliable viewing.

Prioritise unique or irreplaceable records and files with fragile dependencies. A common image format with several verified copies may need less urgent attention than a project database that opens only on one ageing computer.

The NDSA Levels of Digital Preservation provide a staged framework for assessing a preservation programme. Version 2.1, released in March 2026, adds an environmental sustainability focus. Use such a framework to structure proportionate improvements, not as a promise that a particular storage purchase solves preservation. NDSA Levels.

Copies, integrity and readability answer different questions

Copies reduce exposure to a single loss, but copies under the same account or in the same physical location may share a failure. Keep appropriate separation and test restoration from the copy intended for recovery.

A recorded cryptographic checksum can identify changed bytes when checked again. It does not prove the original content was correct, detect every malicious change to the surrounding records or repair a damaged file by itself. Protect the reference records and retain a known-good recovery source.

Readability needs a separate check: open representative files, inspect meaningful content and confirm necessary linked resources. A checksum match on a model with missing external textures does not mean the complete project is usable.

Migrate deliberately and retain evidence

When moving to a new format or system, test a representative sample first. Compare page counts, text, formulas where required, geometry, units, metadata and other important properties. The checks should follow the acceptance statement, not merely confirm that the new file opens.

Keep the original where justified, record the tool and settings, note losses and document approval of the result. A migrated file should have a different checksum; that expected difference is not corruption. The question is whether the meaningful content was preserved according to the agreed criteria.

Do not bulk-convert an entire archive simply because a format is old. Migration can introduce errors, and a well-supported older format may remain usable. For unusual systems, specialist preservation or emulation may be more appropriate than a lossy export.

Put review dates and responsibility in the plan

Review when software is retired, storage is replaced, account ownership changes or a supplier announces an export change. Set a proportionate recurring check as well, with a named owner and a record of restoration and readability results.

Budget for custody, access and occasional migration, including secure continuity of decryption keys. Deletion should follow the relevant retention decision; keeping everything forever can create unnecessary cost and exposure. See Digital account continuity for the people and access side of the plan.