How to Reduce PDF File Size

Most people treat compression as a single button. It is really several different operations that shrink different things inside the file, and which one helps depends entirely on what your PDF is made of.

First: a PDF is a container, not an image

A PDF is a container. It can hold text, vector drawings, embedded fonts, bitmap images, file attachments and invisible metadata. When a PDF is 40 MB, the reason is almost never "page count" — a 300-page contract can be 900 KB while a three-page brochure can be 60 MB.

This is why "PDF compression" is not one operation. Depending on what is inflating your file, one or more of three separate interventions is required.

What actually inflates the size

The practical rule: if your file came from a scanner, the problem is the images. A PDF produced from Word or LaTeX, with selectable text inside, is already small and compression will not buy you much.

A quick way to tell which kind you have

Open the file and try to select text across a paragraph. If words highlight individually, you have a text-based PDF. If the whole page selects as one block, or nothing selects at all, you have an image-based (scanned) PDF.

• Text-based and large: High-resolution images are probably embedded. Target those.
• Image-based and large: Normal. Your gains come from scan resolution and colour depth.
• Text-based and already small: Leave it alone. Do not risk quality to save a few hundred KB.

The DPI decision is the real lever

Scan resolution affects size quadratically. Going from 600 DPI to 300 DPI cuts the pixel count to a quarter. Going from 300 to 150 cuts it to a quarter again. This is where the largest saving almost always hides.

Colour mode: the second lever most people skip

Scanning a black-and-white contract in colour and then trying to compress it gives a far worse result than choosing greyscale in the first place. Colour scanning stores three channels per pixel; greyscale stores one. That change alone usually cuts size to a third and costs nothing in readability.

If you already have a colour scan, converting to greyscale during compression still helps — but making the right choice at the source is always cleaner.

When not to compress

• Official submissions: Some institutions specify a minimum resolution. A compressed file can be rejected.
• Digitally signed documents: Rewriting a signed PDF invalidates the signature.
• Archive copies: Keep the long-term copy at full resolution and produce a separate copy for sharing.
• Files going to print: 150 DPI is visibly bad on paper.

What to realistically expect

A 70–90% reduction is normal for a scanned document. For a text-based file with a few logos, expect somewhere between 5% and 20%. Recompressing an already-optimised PDF often gains nothing, and sometimes degrades quality through JPEG re-encoding while leaving the size unchanged.

When a file refuses to shrink, the reason is usually that its content is already vector and text — there is nothing heavy left to squeeze. If such a file still has to pass through email, consider splitting it instead of compressing it.

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