Types of content
A PDF file of a map, for example, is often a combination of vector graphics, text, and raster graphics. A general reference map of the US uses:
- text stored as such — scalable, and also one can copy the text
- vector graphics for coastlines, lakes, rivers, highways, markings of cities, and Interstate highway symbols — on zooming in, the curves remain sharp, they do not appear as consisting of enlarged pixels (i.e. rectangles of pixels)
- raster graphics for showing mountain relief — on zooming in, this consists of enlarged pixels
To appreciate the difference between vector and raster graphics, compare the general reference map to the CIA World Factbook’s European map. If you zoom in on the former, you’ll see that the (vector) coastline is nicely fitted to the (raster) mountain relief. If you zoom in on the latter, in contrast, the blue of the sea is not nicely fitted to the vector graphics coast line, and the land-sea boundary is much cruder.
Some PDFs have no raster graphics at all. For example, see the Factbook’s map of the Arctic.
Tools exist, such as pdfimages (bundled with Xpdf) to extract the raster images from a PDF file. This can be extremely useful if the PDF is simply a collection of scanned pages.