I am not sure what you mean by "filtered out" - If you save the excel sheet to xml, the link is an attribute of the cell and can be processed to a pdf with xsl-fo.
I created a simple spreadsheet with a cell that contained
www.yahoo.com
this is what it looks like in XML
Code:
<Cell ss:StyleID="s21" ss:HRef="http://www.yahoo.com/"><Data ss:Type="String">www.yahoo.com</Data></Cell>