Why Is My PDF So Large?

What makes a PDF file large?

A PDF file is large because of pictures. Photos and scanned pages hold far more data than text, which a PDF stores as letters and positions. In our tests a page of text took 3 KB, while the same page scanned took 288 KB to 1.9 MB. Fully embedded fonts and old versions kept after edits come next.

Why is a scanned PDF so big?

A scanned PDF is big because each page is a photo of paper, stored pixel by pixel, even if it shows only text. Doubling the scan resolution stores about four times the pixels. Our A4 test page grew from 288 KB at 150 dpi to 1.9 MB at 600 dpi, while the same text as real text took 3 KB.

For ordinary documents, 150 to 200 dpi is enough to read on screen and to print. In our OCR PDF tests, a page scanned at 200 dpi was read with 95% average confidence. Higher settings help for very small print or for archives, at a cost in file size. Making a scan searchable with OCR adds an invisible text layer of a few kilobytes; it does not make the file smaller.

Do photos make a PDF bigger?

Yes. A photo keeps every pixel the camera took unless something reduces it. Our test photo from a 12 MP camera made a one-page PDF of 1.0 MB. Reduced to what an A4 page shows at 150 dpi, the same picture made 216 KB, and at screen size 101 KB. The page looks the same on screen.

A page holds the same picture whatever its pixel count, so extra pixels only add weight. At 756 KB, a 300 dpi copy still suits office printing; 150 dpi suits screens and email. This is the main thing Compress PDF changes, and why it shrinks photo-heavy files the most.

Why do fonts make a PDF large?

A PDF can carry its fonts so text looks the same everywhere. A whole font is heavy: in our test, a one-page file with DejaVu Sans embedded in full was 418 KB. Embedding only the letters the page uses, called a subset, made it 10 KB. A standard font that readers already have needs no embedding: 3 KB.

Fonts with more characters weigh more when embedded in full; DejaVu Sans covers Latin, Greek, Cyrillic and symbols. If your app's PDF export has a font setting, choose to embed only the characters used.

Why did my PDF get bigger after editing?

A PDF editor can save changes by adding them to the end of the file, an "incremental update" in the PDF standard, so the old version stays inside. When we swapped the picture in a 216 KB file this way, it grew to 435 KB: both pictures were stored, one unused. A rewrite brought it back to 219 KB.

Incremental saves are quick and keep digital signatures valid, which is why editors use them. Each round of edits can add more. Compress PDF rewrites the whole file and keeps only what the pages still use, so it removes those old copies.

How do I make a large PDF smaller?

Find the cause first, then fix that. Compress PDF lists how much of your file is images, fonts and page content as soon as you open it. Reduce pictures with its Balanced or Strong level, rescan heavy scans at 150 to 200 dpi, and remove attachments and hidden data you do not need.