為什麼 PDF 檔案會這麼大 — 以及如何縮小它們
PDF 檔案的大小差異極大 — 從純文字合約的幾 KB，到包含嵌入字型與高解析度算圖的工程圖紙的數百 MB。了解原因就能決定如何解決。
依文件類型區分的常見 PDF 大小
Here are realistic file size expectations:
- Simple text contract (5 pages): 50–200KB - Report with charts and some images (20 pages): 500KB–3MB - Scanned document (10 pages, 300 DPI): 3–8MB - Product brochure with high-res photos (12 pages): 10–50MB - Engineering drawing or CAD PDF: 20–200MB - eBook with images (300 pages): 50–200MB
If your PDF is larger than these benchmarks for its type, there is likely room for compression.
導致 PDF 過大的 6 個原因
1. **High-resolution embedded images** — A photo embedded at 300 DPI for print takes up far more space than the 72–96 DPI needed for screen viewing. Reducing DPI is the highest-impact compression technique.
2. **Scanned pages stored as images** — Each scanned page is a photograph. At 300 DPI, an A4 page is roughly 3–5MB before compression.
3. **Unoptimised fonts** — Fully embedding a font family adds 0.5–2MB. Subsetting (embedding only the characters used) reduces this to under 100KB.
4. **Revision history / undo data** — Some PDF creation tools store document revision layers. A file edited 10 times in Adobe Acrobat may retain all 10 versions internally.
5. **Attached files** — PDFs can contain embedded attachments. A 50MB Excel file attached to a 1-page PDF means a 51MB PDF.
6. **Non-optimised export settings** — Printing to PDF from Microsoft Office with 'Best for printing' rather than 'Best for electronic distribution' produces files 3–5× larger than necessary.
壓縮方法及其成效
| Method | Typical size reduction | Best for | |---|---|---| | Image DPI reduction (300→96) | 60–80% | Image-heavy PDFs | | Grayscale conversion | 20–40% | Colour documents where colour isn't needed | | Font subsetting | 5–15% | Fonts-heavy PDFs | | Remove revision history | 10–30% | PDFs edited multiple times | | Linearisation (web optimise) | 2–5% | PDFs served from web servers | | Re-export from source | 40–70% | PDFs created with 'print' export settings |
當壓縮仍不夠時該怎麼辦
If your PDF contains genuinely large content (a 200-page illustrated textbook, a high-fidelity engineering drawing), aggressive compression will visibly degrade quality. In these cases:
- Split the document and distribute only the relevant pages - Share via a cloud link (Drive, Dropbox) instead of attachment - Use a print-on-demand service for physical distribution - Separate the document into a 'text-only' version for distribution and a 'full quality' master for archiving
常見問題
- 正常的 PDF 大小是多少？
- 純文字 PDF 通常為 50–300KB。帶有圖片的報告為 500KB–5MB。在 300 DPI 下，每 10 頁的掃描文件約為 1–10MB。如果您的 PDF 遠大於這些範圍，它可能包含未最佳化的圖片或修訂歷史記錄。
- 為什麼我只有 1 頁的 PDF 會有 10MB？
- 幾乎都是因為它包含高解析度的嵌入圖片（掃描檔、照片或圖表）。在 300 DPI 下，單頁 A4 圖片在進行任何壓縮前就有 5–8MB。使用 WhiskFile 的「小檔案」預設值進行壓縮通常會將其縮減至 200–400KB。
- 頁數對 PDF 檔案大小的影響很大嗎?？
- 會，但與內容成比例。一份 100 頁的純文字 PDF 可能只有 500KB。同樣是 100 頁，如果每一頁都包含一張掃描圖片，大小可能會達到 100MB。頁數的重要性低於每頁的圖片內容。