To rasterise an image means to break it down into dots, rather than lines. The reason you would want to do this is to save resolution; a rasterised image will always look the same, regardless of how big or small it is. PDF images, while very common, may not scale well. They may also not print well, since many professional printers actually use dots to print, and therefore must rasterise the PDF image before printing. These problems may be solved by rasterising a PDF yourself.
Open the image in Acrobat Pro.
Click the "Advanced" tab, and select "PDF Optimizer."
Check the "Off" box in the three "Downsample" windows.
Click "Transparency," then "Flatten Transparency."
Move the "Vector/Raster" slider all the way to "Raster."
Save the new file. Your PDF is now rasterised.
You can only rasterise a PDF with Adobe's Acrobat Pro. Adobe's free Acrobat Reader does not have rasterising capabilities.