Why Is My PDF So Large? (And How to Fix It)

PDF file sizes are wildly inconsistent. A 200-page book can be 2 MB and a three-page scan can be 45 MB. The difference is almost never the amount of content — it is what the content is made of.

The six usual causes

1. It is a scan

Every page is a full-page photograph. At 300 DPI in colour, one A4 page is roughly 25 megapixels of data before compression. This accounts for the majority of oversized PDFs, and it compresses extremely well.

2. Images at print resolution used at thumbnail size

Someone dropped a 12-megapixel photo into a document and scaled it to a 5 cm square. The page shows a thumbnail; the file still contains all 12 megapixels. Downsampling fixes it entirely.

3. Embedded fonts, in full

A complete font family can be several megabytes, and a document using six of them carries all six. Subsetting keeps only the characters actually used and typically recovers most of that.

4. A background on every page

A scanned letterhead or watermark image placed on all forty pages can be stored forty times over. De-duplication collapses it to one copy.

5. Edit history

PDFs support incremental saving: each edit appends to the file rather than rewriting it. A document edited twenty times can carry twenty layers of superseded objects. Rewriting the file discards them.

6. Embedded attachments

PDFs can carry other files inside them — spreadsheets, images, whole documents. They are invisible in the reader and count fully towards the size.

What to do about it

A general-purpose optimiser addresses most of these in one pass, which is why "compress" fixes so many oversized PDFs even when the cause is not obviously an image.

Avoiding it next time

• Scan at 300 DPI greyscale, not 600 DPI colour, for ordinary text.
• Resize images before placing them in a document, not after.
• Export to PDF once at the end rather than saving repeatedly.
• Use two or three typefaces, not eight.