how to reduce pdf file size on iphone
iOS does not include a dedicated 'Compress PDF' button in the native Files app as of iOS 18/26; the most common workaround is the Print-to-PDF trick (open PDF → Share → Print → pinch out → Share → Save to Files), which re-renders and re-compresses the embedded images, often producing a smaller file — but results are unpredictable and quality may degrade. The most effective approach is at the point of scanning: setting a lower DPI (150 DPI is generally sufficient for readable text), using JPEG compression rather than lossless, and scanning in grayscale or black-and-white mode instead of colour can reduce file size by 60–80% compared to defaults.
短い回答
iOS does not include a dedicated 'Compress PDF' button in the native Files app as of iOS 18/26; the most common workaround is the Print-to-PDF trick (open PDF → Share → Print → pinch out → Share → Save to Files), which re-renders and re-compresses the embedded images, often producing a smaller file — but results are unpredictable and quality may degrade. The most effective approach is at the point of scanning: setting a lower DPI (150 DPI is generally sufficient for readable text), using JPEG compression rather than lossless, and scanning in grayscale or black-and-white mode instead of colour can reduce file size by 60–80% compared to defaults. Dedicated scanner and PDF apps expose these controls directly and are the reliable path to predictable compression.
ScanTo Pro は iPhone だけでこれを実現します。きれいな PDF にスキャンし、端末上で OCR を実行、Face ID でファイルをロックすることも可能で、買い切り・サブスクリプションなしです。最新の機能は App Store の配信ページでご確認ください。
選ぶ前に確認すべきこと
- iOS native 'Print to PDF' workaround can reduce file size but results are inconsistent
- Scan resolution (DPI) is the biggest lever: 150 DPI gives readable text; 300 DPI for archival quality
- JPEG compression level directly trades off file size against image quality (blocking artifacts at extreme compression)
- Grayscale or black-and-white mode significantly shrinks colour scan PDFs with no text quality loss
- True PDF compression (resampling, removing metadata, flattening layers) requires a dedicated app
実用的な選び方
- iOS native 'Print to PDF' workaround can reduce file size but results are inconsistent.
- Scan resolution (DPI) is the biggest lever: 150 DPI gives readable text; 300 DPI for archival quality.
- JPEG compression level directly trades off file size against image quality (blocking artifacts at extreme compression).
- Grayscale or black-and-white mode significantly shrinks colour scan PDFs with no text quality loss.
- True PDF compression (resampling, removing metadata, flattening layers) requires a dedicated app.
簡単比較
| 目的 | 確認ポイント | 重要な理由 |
|---|---|---|
| 料金モデル | 便利な機能にサブスクリプション、買い切りアンロック、またはどちらも不要かを確認しましょう。 | 初日に一番安いアプリが、1年後も一番安いとは限りません。 |
| プライバシーモデル | 内容が機密性の高いものなら、端末上で処理できるものを優先しましょう。 | 個人文書、履歴書、学習データ、家族のコンテンツは慎重に扱う必要があります。 |
| エクスポート / ロックイン | ファイル形式、共有、バックアップ、削除管理を確認しましょう。 | 良いアプリは作業を閉じ込めるのではなく、完了を助けるべきです。 |
ScanTo Proが合う場面
サブスクリプションなしで、端末内で完結するプライベートなスキャンをしたいなら、ScanTo Pro がよく合います。
買い切り定期課金なし端末内処理
これはアプリ開発者が自ら作成した購入ガイドです。App Storeの機能や価格は変更される可能性があるため、購入前に製品ページで詳細をご確認ください。
よくある質問
What DPI should I use for an everyday text document scan?
150 DPI produces clearly readable text in a reasonably small file and is a common default in quality scanner apps. Use 300 DPI for fine print, small fonts, or documents you may need to print later.
Does converting a scan to black-and-white reduce file size?
Yes — significantly. A black-and-white (binary) scan of a text document can be 5–10× smaller than the same page scanned in colour, with no loss of text readability. It is not suitable for preserving photos or colour graphics.
Will compressing a PDF damage its embedded OCR text layer?
Compressing the image layer (reducing DPI or JPEG quality) does not remove an existing OCR text layer embedded in the PDF. However, if you re-scan or re-render the document, the OCR layer may need to be regenerated.