I think the xls file may have some hidden elements that are generating page breaks. Can you alter the page length (say, 8.5 by 72) so that the whole thing fits on one long page? Alter in the spreadsheet, and also try altering in the pdf.
Also, you say you "export" to a pdf file. Does "save as pdf" from the print menu give the same results? There's also PDFwriter, a freebie printer driver that you can select from your list of printers; it spools a print job but then sends the output to a pdf file instead of to a physical printer. It gives different results from the "export" command, since it applies printer settings (margins, scaling, paper size, etc.)