Vector vs raster inside a PDF

Zoom into one PDF and the text stays crisp; zoom into another and it blurs. The difference is whether the content is vector or raster.

Vector: math, not pixels

Real text and line art are vector — described by curves and coordinates. They render perfectly at any size and print at full device resolution. This is the PDF at its best.

Raster: a grid of pixels

Scans and photos are raster — a fixed grid of pixels. Enlarge past their native resolution and they soften. A scanned document is entirely raster, which is why it cannot be searched until you run OCR.

Keeping quality high

Keep original text as vector whenever possible — do not "print to image" a document you could export directly. When you must rasterise (for flattening), render at 2× so print output stays sharp.