PDF isn't a image format. Acrobat will store images in a number of formats. Jim, your drawings from Schematics and PSpice are probably stored as CCITT group 4. For line art, CCITT group 4 is hard to beat for black & white. If you have colorful line drawings, then I use ZIP compression of the image in Acrobat's settings with no downsampling.
For word processor documents, my favorite is PNG for line drawings like schematics.
As for your viewing of image files, if you view gif, png, pcx type images at something less than 100% you get aliasing. Some viewers will attempt to anti-alias the image. You can turn off smoothing in Acrobat which speeds up page redraw time at the expense of an aliased image.