General ⇒ Dashes disappear

[Harmont]

When I zoom in/out my document in Yap (dvi) or Acrobat Reader (pdf) I sometimes see en-dashes (--) and sometimes don't. It happens for en-dashes in itemized lists, bibliography ranges (110-115) created by 'cite' package and probably all other cases. Same thing for em-dashes (---).

For instance:
zoom 50% - invisible
zoom 75% - visible
zoom 100% - invisible

It's strange but hyphen (-) is always visible. All types of dashes are visible on the hard paper.

Is there any explanation for that?

Thank you.

P.S. I use default font.