I use 1px border and PDF output generated by PrinceXML is thicker than our current PDF generated by Adobe distiller(? - I am not sure what was used before).

table tr {
border-bottom: 1px solid black;
}

Is there anyway to make a line thinner?

Try using a smaller width, such as 0.5pt for example. However, be aware that the thickness of lines as displayed on the screen can depend on quirks of the PDF viewing application, and you may need to print the file out on paper to see its "true" thickness.

Yes, it works!!! I somehow thought 1px was the smallest unit for the line thickness.