I dropped a pdf containing a 200 dpi grayscale scan into Caboodle. The resulting display is distinctly lower quality than the same file in Acrobat or Preview at (approximately) the same magnification, 75%.
Screenshots on demand.
This site is designed to take advantage of CSS. If you are seeing this, CSS must not be available or enabled in your browser. Everything should still work, but won't be as pretty. :)
Re: PDF appearance
PDFs are rendered by the OS's text engine, so I guess it might scale them to 72 dpi.
If you double-click on the PDF, it'll open in Preview (or Acrobat, depending on your configuration) at full resolution.