One pixel line changing thickness in PDF?

We are  using unstructured FrameMaker 12 on a Windows 7 platform.

We have three uses for 1 pixel horizontal lines: table rows, underscores for hypertext, and horizontal lines on the top and bottom of notes, cautions, tips, etc. The one-pixel lines for the notes and cautions are maintained on the Reference page (like the horizontal line that separates footnotes at the bottom of the page, which we don't use).

We also use conditional text. Right now we have five conditions—all indicated by a different color—not counting the default "black" that represents text that has a universal audience. Due to business reasons, we leave the conditional tags (colors) visible even in our published output.

In all cases described in this post, we are viewing the PDF at 100% magnification.

When we publish to PDF, this is what happens to the thickness of the 1 pixel horizontal lines:

• The hypertext underscores remain 1 pixel when for the black text with no condition indicator. However, the hypertext underscores for conditional text are *always* 2 or 3 pixels thick.
  • If there are two hypertext links within the same blocked condition, the underscore for one of them might be 2 pixels thick, and the next one might be 3 pixels thick.
  • If a hypertext link wraps around to the next line, the underscore might be 2 pixels thick on the upper line, but 3 pixels thick when it wraps to the next line.
• The horizontal lines that set off notes, cautions, tips, etc., usually publish to PDF as 2 pixels thick, regardless of whether or not it is conditionalized.
• The horizontal lines in tables do not seem to change in thickness.

The thickness change indicated by the first two bullets happens regardless of whether we publish to PDF using Acrobat Pro, or using the other PDF tool that is preferred by our company. So, the issue does not seem to be a PDF tool issue. It seems to be the way FrameMaker hands off the content to the PDF tool.

Has anyone else experienced this? Does anyone know if there is a way to make these horizontal lines 1 pixel thick in PDF output?

Thank you in advance!