Sure. There are two basic types of image formats when you break things down. Raster (bitmap for example) and vector (like Illustrator). Raster is a pixel format, which means the image file is a set number of pixels from the start. If you expand a raster file (like a jpeg or bitmap) it becomes all pixelated the larger it gets. What you are seeing is the fact that the file is a set number of pixels, and those squares are limited at the time of the files creation. Vector on the other hand is line art, which means that theoretically one could expand the view of the file and never see any deterioration in the quality of the image. Now if you take a PDF scan and run it into a vector program like Adobe Illustrator it will attempt to translate most of the file into vector art...or to put it this way it will translate the image into lines even without your permission. This is because Illustrator assumes these liberties because the start file is a native PDF format (it will ask if a jpeg or other native raster format). The result is that the file will be rendered into a series of image groups, which one can select below the layer 1 tab and manipulate. Hence because of the way Adobe Illustrator handles a PDF a person can select text and move it around. The white section of the scan is basically just the artifacts that are left over when a person scans an image into a computer. The fact is that it takes a high quality scan (higher than most personal scanners) or a high quality SLR digital camera to be able to effectively capture the date. Although the end result of such a capture would always be left with pixelization because of the way in which computers capture data. Beyond vector art, which really isn't able to be used to capture live images now, we still work with pixels. Programs like Illustrator attempt to compensate for this when they can, but the end result will always be an imperfect capture of the image.
The honest truth is that even if I were to expand Star Wars: Ep II to 500x I would find pixelization on the parts filmed with a digital movie camera. The technology will always have limitations.
So the white pixels around the black lettering is to be expected in a scan of a document into a standard scanner. The scanner works by shinning light in the surface of the document, and it will always leave artifacts. This is another level the birther "experts" are picking up on, but they are expecting near perfect results from an imperfect format.