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Why is my PDF so large? How to find what is taking the space

A PDF is big because of what is inside it, usually photos stored far sharper than the page shows them. How to find the cause on a Mac, measured, and the fix.

A PDF is big because of what is inside it, and the usual cause is pictures stored at far more detail than the page shows. We put one photo on a page two ways: the PDF was 5.26 MB one way and 97 KB the other, from the same picture at the same size on the page.

Why is my PDF so big?

Almost always pictures: scanned pages, or photos placed on the page at camera resolution. Text, even hundreds of pages of it, takes very little room. The usual causes, most common first:

  • Scanned pages: each page is a detailed picture. Three 300 dpi scanned pages came to 7.72 MB in our test.
  • Photos at full resolution: a photo shown 3 inches wide is stored at its full size, however small it looks.
  • Embedded fonts: unusual fonts are packed into the file. Usually small, sometimes not.
  • Saved again and again: some apps add each change to the end of the file instead of rewriting it.

Why is my PDF so big when it only has a few pages?

Because one large photo is enough. We placed a 6016 by 6016 pixel photo 3 inches wide on a single page: the PDF was 5.26 MB. The same photo cut down to 900 pixels, which is 300 pixels per inch at that size and still sharp in print, made a 97 KB PDF. That is 54 times smaller, and it looks the same on the page.

Measured on 15 October 2026 on macOS 27.0.1, with one of the Mac's own desktop pictures saved as a JPEG, placed on a US Letter page.

How do I see what is taking up space in a PDF?

List its pictures. The free pdfimages tool shows every picture with its pixel size and its resolution on the page:

brew install poppler
pdfimages -list document.pdf

Look at the x-ppi column. Around 150 to 300 is normal. Our oversized photo showed 2005, about seven times more detail in each direction than a printer can use. Those pictures are the easiest saving.

Without Terminal, scroll the thumbnails in Preview: pages that are scans or full of photos are where the size is.

How do I make a big PDF smaller?

The one click way

Simply PDF's Compress shrinks each picture to the size it is shown at on the page, so a large photo placed small gives the biggest saving, and leaves text alone. It shows the new size before you save. Compress needs a key; opening and merging are free.

Questions

What makes a PDF file so big?

Usually scanned pages or photos stored at full resolution. In our test, one photo shown 3 inches wide made a 5.26 MB page; resized to what the page shows, it was 97 KB.

How can I see what is inside a PDF?

Preview's Inspector shows basic details, and pdfimages -list in Terminal lists every picture with its size.

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