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For a long-term preservation master, choose lossless compression when keeping the captured image data intact is the priority. Lossy compression can make a PDF smaller by discarding image information, so it is better considered for an access or delivery copy when the size benefit is worth the possible loss of fine detail. A PDF/A label does not, by itself, prove that a scanned document’s original images were preserved losslessly.
What lossless and lossy compression mean for an archive
Lossless and lossy describe how image data is encoded, not whether a PDF conforms to PDF/A. Lossless compression retains the encoded information exactly; lossy compression discards some information. The latter often produces smaller files, but may soften or erase fine features depending on the source and settings.
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The practical trade-off is between fidelity and file size. Compare likely problem areas—small text, line art, stamps, and faint marks—at realistic viewing or print size. There is no universal file-size saving percentage that applies across document types and settings.
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No. PDF/A is a family of ISO standards for constrained PDFs intended to support long-term preservation of page-oriented documents. Conformance does not guarantee that every image stream is losslessly encoded, that the original scan has been retained, or that the file was captured and quality-checked adequately.
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The Library of Congress describes PDF/A-1, PDF/A-2, PDF/A-3, and PDF/A-4. PDF/A-3 permits embedding files in any format, so an archive should consider whether attachments are part of the material it needs to preserve. PDF/A-4 is based on PDF 2.0 (ISO 32000-2:2020). These profiles address different document and workflow needs; no single PDF/A version is universally correct. Library of Congress: PDF/A Family, PDF for Long-term Preservation
When to use each approach
| Consideration | Lossless | Lossy |
|---|---|---|
| Image-data fidelity | Retains encoded information exactly. | Discards information. |
| File size | Often larger for the same content and settings. | Often smaller, with a quality trade-off. |
| Fine detail | Avoids compression-induced loss. | May soften or erase small text, line work, stamps, or faint marks depending on settings and source. |
| Typical role | A conservative choice for a high-fidelity preservation master. | May be suitable for a derivative made for access or delivery. |
The Library of Congress’s format guidance favors lossless compression and high-resolution images among the features of high-quality PDFs, but does not publish a universal benchmark for file-size savings from lossy compression. Library of Congress: PDF Family
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Scanned documents need a source master
When available, retain the original scanned image files as the master and treat a PDF made from them as an access representation unless your preservation policy defines another arrangement. The Library of Congress notes that source images are usually considered the master format for scanned page-image PDFs, and that PDFs made from them may be optimized for access convenience rather than sustainability. Library of Congress: PDF/A Family, PDF for Long-term Preservation
This distinction matters because even a conforming PDF/A can represent a derivative rather than the best available preservation source. A searchable PDF can be useful to readers, but searchability and format conformance do not replace sound capture, quality control, or preservation planning.
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A practical workflow for long-term PDF storage
- Identify the material. Establish whether the collection consists of born-digital PDFs or page images created by scanning; the preservation approach may differ.
- Retain scan masters. For scanned works, keep the source image files when available, together with preservation metadata and integrity controls required by institutional policy.
- Create an access PDF when useful. Choose compression according to the required fidelity and access constraints. If lossy compression is being considered, inspect fine detail at realistic viewing or print size before relying on the derivative.
- Apply PDF/A if required. Select the conformance profile appropriate to the document and workflow, then validate the resulting file with a suitable tool. Validation checks conformance; it does not establish that the source capture was adequate.
- Document the roles of the files. Make clear which files are preservation masters and which are access or delivery copies, so a smaller derivative is not mistaken for the only preserved source.
The Library of Congress lists PDF/A among its preferred formats for textual works and electronic serials. Its 2025–2026 Recommended Formats Statement also lists PDF/A and PDF/UA as preferred page-layout formats and describes high-quality PDFs as having searchable text, embedded fonts, lossless compression, and high-resolution images. These are format preferences, not a claim that every archive must use one PDF/A profile or compression setting. Library of Congress: Recommended Formats Statement 2025–2026
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How to decide for a particular collection
- Fidelity is the priority: favor lossless compression for a PDF preservation master, and retain the scan source files when available.
- Convenient access or smaller delivery files are the priority: a lossy derivative may be reasonable if its detail remains fit for the intended use and the master is preserved separately.
- PDF/A is required: choose and validate an appropriate profile, while separately checking the compression, source files, quality, and preservation responsibilities.
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