Why Is My PDF So Large — and What Actually Compresses It?

Your PDF is large because it embeds photos or scans at 300+ DPI in JPEG or lossless PNG. Text and vectors barely add up; images do. The fix is to downsample images to screen DPI (150), re-encode with modern JPEG/JBIG2, and enable object streams. Zro7 Compress PDF does all three, locally.

The three sources of bloat

1. Raster images at print DPI — often 5–20 MB per full-page image at 300 DPI.
2. Uncompressed streams — many exporters skip FlateDecode, doubling the size.
3. Embedded fonts — full font sets when only a few glyphs are used.
4. Duplicate objects — many PDFs re-embed the same logo/header on every page.

What actually shrinks a PDF

• Downsample images to 150 DPI (screen) or 96 DPI (email). Roughly 4× size reduction each halving.
• Re-encode with quality 75 JPEG for photos; JBIG2 for B/W scans (10–30× on scanned pages).
• Rewrite the file with FlateDecode + object streams.
• Subset embedded fonts.

Lossless vs lossy — pick the right mode

• Lossless — object streams, FlateDecode, dedup. Usually 10–30% smaller. Safe for archival.
• Lossy — plus image downsampling / re-encoding. 50–90% smaller. Best for email, web.

Zro7 vs Adobe Acrobat compression

• Adobe — mature presets ('Standard', 'Smallest File Size'), server-optional.
• Zro7 — free, browser-only, comparable results on scans; requires a modern browser.
• Both can produce a 2 MB output from a 50 MB scan-heavy PDF.

Steps

1. Open Compress PDF.
2. Drop the PDF.
3. Choose preset (Screen / eBook / Print / Prepress).
4. Download; compare sizes.

Updated December 24, 2026 · Zro7 editorial team.