為什麼我的 PDF 檔案這麼大？ — 原因與解決方法

**1. High-resolution photos and images.** The most frequent cause. A photo embedded at 300+ DPI (print quality) is 4–9× larger than the same photo at 72–150 DPI (screen quality). A document with 10 full-page photos at 300 DPI can easily reach 30–50 MB.

**2. Fully embedded fonts.** When a PDF embeds complete font files (for maximum compatibility), each font adds 100–500 KB. Documents using many fonts can add several MB in font data alone.

**3. Scanned pages stored as high-resolution TIFF images.** Flatbed scanners default to 300–600 DPI TIFF format. A 10-page scan at 300 DPI TIFF can easily be 30–50 MB before any other content is added.

**4. Revision history and undo data.** When a PDF is edited repeatedly in Adobe Acrobat or similar tools, each edit session is appended to the file (not overwriting it) to preserve undo history. A simple 2 MB document can grow to 15 MB after extensive editing.

**5. Attached files.** PDFs can have files attached inside them (like email attachments). Check the Attachments panel in a PDF viewer if a file seems unreasonably large.

**6. Unoptimised export settings.** When exporting from Word, Illustrator, or InDesign with default or print-quality settings, images are often retained at full resolution even when the PDF will only be viewed on screen.

The fastest approach is to use the WhiskFile Compress PDF tool, which automatically identifies and reduces the largest contributors to file size:

- Images are downsampled from print DPI (300+) to screen DPI (72–150) - Duplicate image data is deduplicated - Unused font subsets are trimmed - Revision history and update layers are merged

Most PDFs can be reduced by 40–70% in a single pass.

For maximum compression, use the 'Small' compression level. For a balance of size and visual quality, use 'Balanced'.

PDF files are designed as append-only: when you edit a PDF, the original content is not overwritten. Instead, new data is appended and the original is marked as obsolete. This preserves undo history but causes the file to grow with each edit session.

Over time, a repeatedly edited PDF accumulates many layers of obsolete data. Running it through a PDF compressor removes this accumulated bloat by linearising the file — merging all layers into a single optimised version.

Scanners produce high-resolution raster images (TIFF or JPEG) of each page. A single page scanned at 300 DPI produces a TIFF image of roughly 3–8 MB. A 20-page document scanned at 300 DPI can be 60–160 MB before any PDF compression.

Two solutions: 1. Compress the scanned PDF to downsample images to screen resolution. 2. Apply OCR and then compress — OCR adds a text layer, and modern PDF compressors can then treat the page differently (text is stored much more efficiently than pixels).