目次
PDFとPDF/Aは似ているように見えますが、異なる目的に使用されます。文書のアーカイブ、法的遵守、そしてファイルが将来にわたってアクセス可能であることを保証するために、各形式をいつ使用すべきかを理解することは重要です。
If you have ever received a PDF that looked different on someone else's computer, or been told by a government agency that your submission must be in "PDF/A format," you know the confusion surrounding PDF variants. This guide demystifies the difference between standard PDF and the PDF/A archival format, explains every version and conformance level, and shows you exactly how to convert between the two.
PDFとPDF/Aとは？
両形式は同じ基盤を共有していますが、異なる目標と制限を持っています：
標準PDF
Portable Document Format（PDF）は文書交換のために設計されています。フォーム、マルチメディア、JavaScript、外部リンクなどのインタラクティブな機能をサポートします。PDFはビューアーのソフトウェアや利用可能なフォントによって見た目が異なる場合があります。
PDF/A（アーカイブ）
PDF/Aは長期保存のために設計されたISO標準化バージョンです。必要なすべてのリソースをファイル内に埋め込み、将来レンダリングの問題を引き起こす可能性のある機能を禁止しています。
PDF/Aは2005年にISO 19005-1として最初に公開されました。10年、50年、または100年後でも、文書が今日と全く同じように見えることを保証します。
PDFとPDF/Aの主な違い
2つの形式の詳細な比較：
| 機能 | PDF/A | |
|---|---|---|
| フォント埋め込み | 任意 | 必須 |
| JavaScript | ✅ | ❌ |
| 暗号化 | ✅ | ❌ |
| 外部参照 | ✅ | ❌ |
| XMPメタデータ | 任意 | 必須 |
| カラースペース | 任意 | デバイス非依存 |
PDF/Aバージョンの説明
PDF/Aはいくつかのバージョンを経て進化し、それぞれ新しい機能が追加されています：
PDF/A-1
PDF 1.4に基づく。厳格な要件を持つオリジナル規格。サブセット：PDF/A-1a（アクセシブル）とPDF/A-1b（基本）。
PDF/A-2
PDF 1.7に基づく。JPEG2000、透明度、埋め込みPDF/Aファイルのサポートを追加。現在最も広く使用されているバージョン。
PDF/A-3
PDF/A-2と同じですが、任意のファイルタイプ（Excel、XMLなど）を添付ファイルとして埋め込むことができます。
PDF/A-4
PDF 2.0に基づく。簡素化された適合性レベルとモダンな機能を持つ最新バージョン。
Conformance Levels Explained
Each PDF/A version comes in different conformance levels, designated by a letter suffix. Understanding these is important when an organization specifies a particular requirement like "PDF/A-2b."
Level b (Basic)
The minimum conformance level. It ensures the visual appearance of the document is preserved long-term. Fonts are embedded, prohibited features are removed, and color spaces are standardized. However, Level b does not guarantee that text can be reliably extracted or searched -- the focus is purely on visual reproduction.
Level a (Accessible)
Builds on Level b by requiring the document to be "tagged" with a logical structure. This means screen readers can interpret headings, paragraphs, tables, and reading order. Level a is essential for organizations that must meet accessibility standards (like Section 508 in the US or EN 301 549 in the EU). Creating a Level a document requires more effort during authoring, as the document structure must be explicitly defined.
Level u (Unicode)
Introduced in PDF/A-2, Level u sits between b and a. It requires that all text in the document maps to Unicode characters, ensuring text can be searched and extracted reliably. However, unlike Level a, it does not require full structural tagging. This makes it a practical middle ground for organizations that need searchable archives but do not require full accessibility compliance.
When in doubt, target PDF/A-2b. It is the most widely accepted conformance level, supported by all major archival systems, and does not require the complex structural tagging that Level a demands.
各形式の使用タイミング
標準PDFを使用する場合：
- 即時使用またはコラボレーションのために文書を共有する場合
- フォームやマルチメディアなどのインタラクティブ機能が必要な場合
- ファイルサイズが重要な場合（PDF/Aファイルは通常大きくなります）
- パスワード保護などのセキュリティ機能が必要な場合
PDF/Aを使用する場合：
- 長期アーカイブまたは保存が必要な場合
- 法的または規制遵守で必要な場合
- 文書が数十年後も読み取り可能である必要がある場合
- 政府や医療のための公式記録を作成する場合
How to Validate PDF/A Compliance
Just because a PDF looks correct does not mean it conforms to the PDF/A standard. Validation is the process of checking whether a file meets all the requirements of a specific PDF/A version and conformance level.
Why Validation Matters
Many organizations -- particularly government agencies, courts, and archives -- reject non-compliant files outright. If you submit a PDF that claims to be PDF/A but fails validation, your submission may be returned. Validating before sending avoids delays and frustration.
Validation Tools
- Adobe Acrobat Pro: Go to View > Navigation Panels > Standards, and click "Verify Conformance." Acrobat will report any violations found.
- veraPDF (free, open source): The industry-standard validator for PDF/A. It supports all versions and conformance levels and produces detailed compliance reports. Available at
verapdf.org. - Online validators: Several web-based tools can check PDF/A compliance by uploading your file. These are convenient for occasional checks.
Common Validation Errors
The most frequent issues that cause PDF/A validation to fail include:
- Missing embedded fonts: Every font used in the document must be fully embedded. System fonts referenced but not embedded will cause a failure.
- Transparency issues: PDF/A-1 does not support transparency. If your document uses semi-transparent elements, you must either flatten them or target PDF/A-2 or later.
- Missing XMP metadata: PDF/A requires XMP (Extensible Metadata Platform) metadata in the document. Some PDF generators omit this.
- Forbidden annotations: Certain annotation types (like 3D content or file attachments in PDF/A-1) are not permitted.
Common PDF/A Conversion Issues
Converting a standard PDF to PDF/A is not always seamless. Here are the issues you are most likely to encounter and how to resolve them:
Color Space Problems
PDF/A requires device-independent color spaces (like sRGB or ICC-based profiles) instead of device-dependent ones (like DeviceRGB). If your source document uses device-dependent colors, the conversion tool must add an appropriate ICC profile. Most conversion tools handle this automatically, but the colors may shift slightly in the process.
Interactive Form Fields
PDF/A-1 and PDF/A-2 do not support JavaScript, which means form fields that rely on scripts will lose their functionality. Static form fields (dropdowns, text inputs, checkboxes) are preserved in PDF/A-2 and later, but any calculated fields or validation scripts will be stripped. If your document has interactive forms, complete them before converting to PDF/A.
Embedded Multimedia
Audio and video content is not permitted in PDF/A-1 or PDF/A-2. PDF/A-3 allows embedded files of any format but does not guarantee they will be playable. If your document contains multimedia, you will need to remove it or replace it with static content (screenshots, transcripts).
File Size Increase
PDF/A files are typically larger than their standard PDF counterparts because fonts must be fully embedded and certain compression techniques are restricted. Expect a 10-50% size increase. If the resulting file is too large, use the PDF compressor afterward -- just ensure the compression tool preserves PDF/A compliance.
PDFをPDF/Aに変換する方法
PDF-Ninjaを使用すれば、通常のPDFをPDF/Aに簡単に変換できます：
- PDFファイルをPDF/A変換ツールにアップロード
- 必要なPDF/Aバージョンを選択（ほとんどの用途にはPDF/A-2bを推奨）
- 変換をクリックして処理を待つ
- 準拠したPDF/Aファイルをダウンロード
After conversion, always validate the output as described in the validation section above. Some conversion tools produce files that visually appear correct but fail strict compliance checks.
The best PDF/A documents start their life as PDF/A. If possible, configure your source application (Word, InDesign, LibreOffice) to export directly to PDF/A rather than creating a standard PDF and converting it later. This avoids most conversion issues entirely.
結論
PDFとPDF/Aのどちらを選ぶかは、特定のニーズによります。日常的な文書共有には、標準PDFがより柔軟性を提供します。アーカイブ、法的遵守、または長期保存には、PDF/Aが文書を数十年にわたってアクセス可能で変更されないことを保証します。無料の変換ツールを使用すれば、必要なときにいつでも準拠したPDF/Aファイルを簡単に作成できます。
Whether you are archiving corporate records, submitting legal filings, or preserving historical documents, understanding the differences between PDF and PDF/A empowers you to make the right format choice. For most archival needs, PDF/A-2b strikes the best balance between compatibility and ease of creation. Use the PDF to PDF/A converter to get started, and explore our related guides on compressing PDFs and electronic signatures.
Frequently Asked Questions
Can I convert a PDF/A back to a regular PDF?
Technically, yes. Opening a PDF/A in most editors and re-saving it as a standard PDF removes the PDF/A restrictions. However, this defeats the purpose of archival formatting. If you need to edit a PDF/A document, edit it and then re-convert to PDF/A when done.
Does PDF/A support digital signatures?
Yes. PDF/A-2 and later versions support digital signatures, including PAdES (PDF Advanced Electronic Signatures). This makes PDF/A suitable for legally signed documents that need long-term archival. See our guide on signing PDFs legally for more details.
Which PDF/A version should I use?
For most purposes, PDF/A-2b is the recommended choice. It supports JPEG2000 compression and transparency (unlike PDF/A-1), is widely supported by archival systems, and has a lower creation barrier than Level a conformance. Only use PDF/A-1 if explicitly required by a regulatory body.
Why is my PDF/A file larger than the original PDF?
PDF/A requires all fonts to be fully embedded and prohibits certain compression methods. These requirements increase file size by 10-50% compared to a standard PDF. You can use PDF compression tools afterward, but verify that the compressed file still passes PDF/A validation.
Is PDF/A the same as PDF/X or PDF/E?
No. PDF/A is for archiving, PDF/X is for print production (ensuring reliable printing), and PDF/E is for engineering documents. Each is an ISO standard subset of PDF optimized for a specific use case. They share the same base format but have different restrictions and requirements.