如何壓縮 PDF —免費線上指南

Large PDF files are slow to email, slow to upload to cloud storage, and fail attachment size limits on platforms like Gmail (25 MB) and Outlook (20 MB). Compressing a PDF fixes all three problems without changing the document's content.

The most common causes of large PDFs are high-resolution photos embedded at print quality (300+ DPI) when screen resolution (72–150 DPI) is sufficient, redundant font embedding, and unoptimized metadata. A PDF compressor strips out this excess data automatically.

1. Go to the WhiskFile Compress PDF tool.

2. Click "Choose file" or drag and drop your PDF onto the upload zone. Files up to 50 MB are accepted on the free plan.

3. Select a compression level: Small (maximum reduction, lower image quality), Balanced (recommended for most files), or Quality (minimal reduction, best image fidelity).

4. Click "Compress PDF" and wait a few seconds. Processing time depends on file size — most PDFs complete in under 10 seconds.

5. Download the compressed file. The download page shows the original size, compressed size, and the percentage saved.

Use "Balanced" for most documents — it typically reduces file size by 40–60% while keeping images looking sharp on screen and in print.

Choose "Small" when you need to hit a specific size target (such as an email attachment limit) and the document contains mostly text with some images.

Choose "Quality" when the PDF contains professional photography or artwork that must be printed at high resolution, and you only need a moderate size reduction.

If the PDF was exported from Microsoft Word or Adobe InDesign, re-exporting with "screen optimized" or "smallest file size" settings before compressing can dramatically reduce the starting size.

For scanned PDFs, OCR (Optical Character Recognition) can sometimes reduce file size by converting image layers to searchable text layers. Try the WhiskFile OCR PDF tool first.

If your PDF has many pages, compressing and then splitting into smaller documents can help when sharing with recipients who have strict email limits.

PDF compression is safe and reversible — the original file is not changed. WhiskFile processes your file on secure servers and automatically deletes it after 1 hour.

Image quality is affected by compression level. At "Small", photos may appear slightly pixelated when zoomed in at 200%+, but look normal at 100%. Text, vector graphics, and charts are never degraded by PDF compression because they are stored as scalable data, not pixels.