How to compress a PDF without losing quality

A PDF that's suddenly too big to email, too slow to upload, or rejected by a form with a size limit is one of the most common file problems people run into, and one of the most misunderstood. "Compress PDF" tools often hide what they're actually doing to your file, and the wrong setting can quietly turn searchable, selectable text into a flat image. This guide covers what actually makes a PDF large, the two genuinely different ways to shrink one, and how to pick settings that get the size down without wrecking legibility.

What actually makes a PDF file large

Text itself is tiny, a hundred pages of typed text might only add up to a few hundred kilobytes. The size almost always comes from one of three places:

• Embedded images. Scanned pages, screenshots, or high-resolution photos dropped into a document. This is the single biggest contributor for most oversized PDFs, especially anything produced by a scanner or a phone's "scan to PDF" feature.
• Embedded fonts. PDFs often bundle the full font files used in the document so it looks identical on every device. A handful of embedded font families can add a meaningful chunk of size, particularly in design-heavy documents.
• Inefficient internal structure. PDFs edited or re-saved multiple times can accumulate redundant or poorly organised internal data, old versions of objects that are no longer used but never got cleaned out.

Two genuinely different kinds of "compress"

Our Compress PDF tool offers both, labeled honestly rather than hidden behind a single "compress" button:

• Quick clean re-saves the PDF's internal structure more efficiently without touching a single pixel of content. Text stays selectable and searchable. This targets the third cause above, inefficient structure, so the gains depend on how "messy" the original file already was. A PDF exported cleanly from one source might barely shrink; one that's been edited and re-saved several times can shrink meaningfully.
• Strong compression re-renders every page as a compressed image at a resolution and quality you choose. This directly targets the first cause, embedded images, and can shrink a scanned or photo-heavy PDF dramatically. The tradeoff is real: text becomes part of the image and stops being selectable, copyable, or searchable.

Picking resolution and quality settings

If you go with strong compression, the two numbers that matter are resolution (measured in DPI, dots per inch) and image quality (a percentage). As a starting point:

• 96–120 DPI is enough for a document that will only ever be read on a screen. This is where the biggest size savings live.
• 150 DPI is a reasonable middle ground, noticeably sharper than 120 DPI if the document does get printed occasionally, without ballooning the file size.
• 300 DPI is the standard for anything intended for quality printing or archival. Expect a much larger file at this resolution.

For quality, 70–85% on a JPEG-style scale is typically close to visually lossless for document scans, text and line art stay crisp, and you'd need to zoom in significantly to spot artifacts. Dropping much below 60% starts to show visible blockiness around text edges, especially at lower DPI.

Step by step

1. Open Compress PDF and add your file.
2. Try Quick clean first if the document is mostly typed text, it's the lower-risk option and keeps everything searchable.
3. If the file is still too large, or it's a scan/photo-heavy document, switch to Strong compression and start around 120 DPI / 70% quality.
4. Download the result and open it, check that text you need is still legible (or still selectable, if that matters for your use case) before you send it anywhere.

Frequently asked questions

Related guides

• How to Merge PDF Files Online for Free
• How to compress an image for web or email