This is a known effect of adding transparency to a page. It will show on screen and in low-resolution printed output, but generally not on press.
There are several reasons possible:
- Fonts have been converted in InDesign. Don't do that.
- Transparency in some kind and form
- Stroke applied to the font in the original file
I would recommend to inform your clients on specifications, how the PDF should be exported. My recommendation for best PDF import are:
- Bleed (I ask for 4mm on each side)
- No font is allowed to be outlined.
- Don't use EPS or PDF/X-1a or PDF/X-3 or any PDF with flattened transparency
- Use for images RGB.
If the PDFs you get are created according to these point you will receive PDFs for the best quality.
Also, the transparency "bolding" preview effect is apparently an OS problem—I've never seen it on OSX. What's your OS? If you export the pages back to PDF do you see the problem in AcrobatPro or Reader?
Interesting comment about OS specific. I've never seen anyone mention that before. I'm a Windows user, and I do see the effect on screen, and sometimes in laser prints, with PDFs, and we've historically seen many complaints about that. Given the number of Mac users, I find it hard to believe that all the complaints are from the PC side, and i think you are just lucky.
In my experience the only thing putting text above transparency accomplishes is preventing rasterization during flattening (which is definitely important), but never preventing the "emboldening" effect.