PDFを100KB未満に圧縮する方法

Getting a PDF under 100 KB is a tough target. Most standard PDF compression gets files to 20–40% of their original size, which is great — but if you're starting with a 2 MB PDF, you'll land around 400–800 KB, not 100 KB. Reaching that 100 KB threshold requires understanding why the PDF is large and attacking each cause directly.

Is 100 KB Realistic for Your PDF?

Before trying, check what's actually in your PDF:

| PDF Type | Typical Compressed Size | Can Reach 100 KB? | |---|---|---| | 1-page text document | 40–80 KB | Likely already there | | 2-page form with logo | 80–150 KB | Yes, with optimization | | 5-page report with charts | 200–500 KB | Yes with aggressive compression | | Photo-heavy brochure (6 photos) | 500 KB – 2 MB | Possibly, at lower image quality | | 50-page scanned document | 5–50 MB | No — split into parts | | High-res presentation (slides) | 10–100 MB | No — needs splitting + optimization |

If your PDF is a scanned document or a slide deck, 100 KB is almost certainly not achievable as a whole file. The strategies below will get you as small as possible, but some PDFs have an irreducible minimum.

Step 1: Standard Compression First

Start here before trying anything else.

1. Go to the Compress PDF tool.
2. Upload your PDF.
3. Select Small (maximum compression).
4. Download the result and check the file size.

Expected reductions:

• Text-only PDFs: 10–30% (already small, minimal gains)
• Office documents exported to PDF: 30–60% reduction
• Photo-heavy PDFs: 50–80% reduction

If the result is under 100 KB, you're done. If not, continue to Step 2.

Step 2: Convert Color to Grayscale

Color images are 3–4× larger than grayscale equivalents. If your PDF contains color photos or graphics and doesn't need to be in color, converting to grayscale can cut image file size dramatically.

1. Use the Grayscale PDF tool.
2. Upload your compressed PDF from Step 1.
3. Download the grayscale version.
4. Run through Compress PDF one more time — grayscale images compress even further.

A 400 KB color PDF can often become 80–120 KB as grayscale with full compression applied.

Step 3: Reduce Page Count

Every page adds size. If your PDF has pages you don't strictly need:

1. Use the Organize PDF tool to delete unnecessary pages.
2. Remove blank pages, duplicate pages, or pages with content that doesn't need to be in this file.

For multi-page reports where you only need a specific section, use Extract PDF Pages to create a smaller file containing only the pages you need.

Step 4: Use Resize PDF to Reduce Dimensions

If your PDF is formatted for large paper (A3, tabloid, poster-size) or uses unnecessarily large page dimensions, resizing to A4 or Letter reduces image rendering size proportionally.

1. Use the Resize PDF tool.
2. Select A4 (or Letter for US).
3. This resizes the page canvas, which causes embedded images to be rendered at lower effective resolution.
4. After resizing, run Compress PDF again.

Step 5: Split and Send Parts

If you need to stay under 100 KB for a specific upload form or system requirement but the full document won't fit, split it into parts.

1. Use Split PDF by Size.
2. Set the maximum size to 95 KB (with buffer).
3. The tool produces multiple files, each under your limit.
4. Upload or send each part separately.

This is the right approach for large scanned documents where individual-page compression is already optimized.

The Source Document Strategy

If you're regularly creating PDFs that need to be small, fix the problem at the source:

From Microsoft Word:

• File → Save As → PDF → Options → "Minimum size (publishing online)"
• Remove embedded fonts for common fonts (Arial, Times New Roman, Calibri are available on all systems)
• Resize images in the Word doc to the actual display size before inserting them (don't insert a 5 MB photo and let Word/PDF scale it down — scale first in an image editor)

From Google Docs:

• File → Download → PDF — Google uses reasonable compression by default
• Remove embedded images or reduce their quality before downloading

From scanning:

• Use 150 DPI, grayscale mode, JBIG2 compression for black-and-white documents
• Use 200 DPI for documents that need some image quality
• Never use 600 DPI for document scanning unless you're archiving for print

Quick Answers

My PDF is 2 MB of scanned text. Can I get it under 100 KB? Unlikely as a single file. A standard scanned page at 200 DPI is ~80–150 KB. A 20-page scanned document will be 1.5–3 MB even after optimization. Split into individual pages if a hard limit is required.

Can I compress a PDF below its theoretical minimum? No. Once all redundant data, metadata, and uncompressed streams are removed, the remaining content (image pixels, font data, vector graphics) has a physical minimum size. Compression at this point would visibly degrade image quality.

Does compression affect text quality? No. PDF compression primarily targets image data. Text that is stored as vectors (which is the case for all digitally-created PDFs) is unaffected by compression and remains crisp at any zoom level.

What's the smallest a 1-page text-only PDF can get? A simple 1-page text document with no images or formatting can compress to 8–25 KB. Very minimal PDFs (single-page, plain text) can sometimes reach 4–6 KB.