Supposedly Identical characters show up differently in PDF

Question

I applied the same paragraph style and set the character style to [None]; In InDesign the characters look identical. But when I generate a PDF with the common presets the two pages come out distinct. These differences stay consistent through the whole page and the undesired character thickining only happens on pages with images; although only with a small fraction of images.
See http://imageshack.us/photo/my-images/607/weirdw.gif/

Answer 1

Trnasparency comes from multiple sources, andyou may not realize that you are using it.
Images and graphics often use transparent areas. That's a form that most users would recognize (and usually want -- do you want to see white boxes around things you place in front of a colored background?). Likewise, you can set an object to be transparent under the effects panel to allow something behind it to show through. But perhaps the largest source of transpaency is the use of blending modes and effects in ID which are not obviously transparency, drop shadows for example, or a glow.
You cannot globally turn off transparaency, nor can you make a transparent image non-transparent without editing and resaving it. You can avoid use of shadow, effects, blend modes and applied transparency within ID, though.
The "emboldening" of type is generally not a problem for high-end printing, but it can be annoying fro low resolution prints of on-screen display. The best you can do if you need transparency in your file, and you don't want to see any differnce between pages with and without, is to be sure there is transparency on all pages. Yo can do that by adding something to the master pages.