Version:           0.9.5 (using KDE 4.4.4) 
OS:                Linux

Unfortunately in Okular 0.9.5 as it comes with KDE 4.3.5 "release 0" (same also in KDE 4.3.5 "release 3") from openSuSE I found very annoying the under line tool for annotations.
The line it produces is too thick and actually is not not placed under the letters but goes directly over the bottom part of the letters.

This makes very difficult to read in places where underline annotation are used. Especially if the original text is written with small letters it is a real problem to understood the meaning of the underlined text.

My suggestion is possibly to make this line little bit not so thick and possibly place it some dots below the current position in order to make the
underlined places more readable.

Reproducible: Always

Steps to Reproduce:
1. Open any PDF file.
2. Press F6
3. Choose the underline tool
4. Apply it on the text. 

Actual Results:  
You will get a very thck line comvering 1/3 to 1/2 from the letters instead to go below and behind them.

Expected Results:  
The line to be more thin to be placed behind and below the text

1. Possibly to make the line not so thik 
2. To place the line behind the text (as the yellow marker tool is doing it)
3. To place the line little bit lower - below the letters or may be on the botom edge of the letters.
4. This is to prevent any hidding of the letters by the underline