Vector vs Raster: How Images Work in PDFs

Vector graphics

A vector graphic is a set of drawing instructions: “line from here to here”, “circle with this radius”, “fill with blue”. Text, logos, charts and diagrams exported from Word, Excel, PowerPoint or design software are usually vectors.

• ✅ Perfectly sharp at any zoom level
• ✅ Tiny file size
• ✅ Text inside can remain selectable

Raster images

A raster image is a grid of coloured pixels: photos, screenshots and every page of a scanned document.

• ❌ Becomes blurry or blocky when enlarged
• ❌ File size grows with resolution and colour depth
• ✅ The only way to store photographs

Resolution (DPI) in plain English

DPI (dots per inch) is how many pixels are packed into each inch of the printed page.

DPI	Typical use	Approx. size of one colour A4/Letter page
72–96	Screen previews	50–150 KB
150	On-screen reading, email	150–400 KB
300	Office printing, OCR	0.5–1.5 MB
600	Archival scans, fine print	2–6 MB+

(Sizes vary widely by content and compression.)

How images are compressed inside a PDF

• JPEG (DCT): Great for photos, lossy. Lower quality = smaller file.
• Flate (ZIP): Lossless, good for screenshots and graphics with flat colours.
• JBIG2 / CCITT: Very efficient for black-and-white scans.
• JPEG 2000: More efficient than JPEG but less widely used.

Compression tools mainly shrink PDFs by lowering image resolution (downsampling) and re-compressing images. See How PDF compression works.