ทำไมไฟล์ PDF ถึงมีขนาดใหญ่ — และวิธีลดขนาด

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.

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.

| 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 |

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