Raster image

A raster image is a rectangular grid of pixels, each holding a colour value, the model behind every photograph and scan. Unlike a vector, a raster has a fixed native resolution: it stores exactly so many dots across and down, and all its detail is baked into that grid.

The practical limit follows directly. Enlarge a raster beyond its native size and there is no extra information to draw on, so the renderer invents pixels by interpolation and the image goes soft or blocky. This is why a small web image looks rough when blown up to poster size, and why scan resolution, measured in DPI, decides how much you can zoom or print. Common raster formats inside and around PDFs include JPG for photos, PNG for sharp-edged graphics, TIFF for archival scans and WebP for efficient web delivery.

When a PDF page is exported as an image, or a photo is placed into a PDF, you are working with raster data, and choosing the right resolution and format up front is what keeps quality and file size in balance. Doing those conversions locally keeps the source images on your own device.