Why does some text look bold and some normal weight in the tiff?

 

This typically happens with text that is vectorized in the PDF.  AutoCAD SHX text is almost always vectorized and most PDF drivers except AcroPlot will vectorized the True Type Text also.  PDF files are creates at a specific DPI and you are converting them to another DPI for the TIFF.  In this, it's possible that lines do not end up being the exact same width as what they were supposed to be due to rounding errors during the rendering process. To minimize this you should always create the PDF at the same resolution as what you are planning on plotting it to. In the conversion to TIFF at say 400 dpi, you have 1/400 or 0.0025 inch resolution.  That means that a line can be 0.0025" in thickness, 0.0050", 0.0075" and so on.  If the PDF calls for a line to be 0.0060" then it's typically going to be rounded down to 0.0050".  When you start taking into account the position on the page for the line and those rounding errors you can see what happens.