I generated a pdf file from a word document with word 2007. But the pdf file looks very uncomfortable in my pdf viewer. So I looked it closely to see what makes me feel so. The culprit is the spacing between characters. As shown in the image blow, the spacing between characters within a word varies in the pdf file, though the original word document doesn't exhibit this.
I tried to export the pdf file with openoffice again. To my surprise, the generated file looks very smooth and comfortable. The spacing between characters is perfect, just as they are in doc file. Also, the edge of the font is much more clear.