如何在線上將 PDF 壓縮至 1MB 以下 — 免費指南

Many email clients (especially corporate servers) reject attachments over 10MB. But form submission portals, insurance claim uploads, job application sites, and government agencies often impose a much stricter 1MB or 2MB cap. The 1MB limit is common enough that it is worth understanding how to hit it reliably.

Upload your PDF to WhiskFile's Compress PDF tool and select the 'Small file' preset. This applies aggressive Ghostscript compression and reduces image DPI to 72–96, which is sufficient for screen reading. For a typical 5–10MB document with photos, this alone usually achieves 60–80% reduction.

If your file starts at 3MB or less, this step will almost certainly get you below 1MB.

If your PDF contains colour images but colour is not essential (e.g. scanned forms, text documents with coloured headers), convert it to grayscale before compressing. Grayscale images are roughly 1/3 the size of equivalent colour images.

Use WhiskFile's Grayscale PDF tool, then compress the result. This combination frequently achieves 70–85% size reduction.

Scanned PDFs are the hardest to compress because each page is a high-resolution photograph. If compression alone is not enough:

1. Use WhiskFile's Compress PDF with 'Small file' preset 2. If still over 1MB, try reducing to fewer pages (split and process the critical section only) 3. For new scans: scan at 150 DPI rather than 300+ DPI — files will be 4× smaller with acceptable readability

If the document has blank pages, cover pages, or sections not needed for submission, delete them first. Fewer pages = smaller file. Use WhiskFile's Delete PDF Pages tool to remove specific pages, then compress the result.

If your compressed PDF is still over 1MB, the document likely contains very high-resolution embedded images that resist compression. Options:

- Split the document and submit in parts (if the portal allows) - Ask the receiving party if they accept ZIP files (you can ZIP a 2MB PDF down to ~1.8MB) - Re-scan the document at 100 DPI (text remains readable, file size drops dramatically) - For scanned multi-page documents: 1MB ÷ number of pages = your per-page budget. At 150 DPI, a typical A4 page scans to 50–80KB.