Why does an Adobe Illustrator document saved as a PNG that are then converted to PDF files look blurry and pixelated?
The image looks pixelated because the vector artwork is being rasterized. Edges that don't fall on whole pixels are anti-aliased.
Unless there is a specific reason to save as PNG, and I can't imagine what that would be, you should save directly to PDF to retain the vector information. Beyond having a sharper image, vector art usually produces smaller file sizes.