it cant be done. google for pdf in thickbox. a while back i needed that and had to manually modify the thickbox.js to allow it to embed a pdf. even after i got it to work it was not 100% sometime all it would load was a red X. why not create a jpg of the pdf at 100% and put that into a scrolling textfield. it would be very close to the same thing. place a link in the textbox to allow direct downloading of the pdf if the jpg is not good enough. i just know for sure showing a pdf in thickbox is not doable without a hack of the js.
i did some digging and found my old tour where i used this, like i said it was not a 100% fix when it did work and i think FF does not support this at all. so i already undid my use of pdfs in that tour but there was 1 thickbox files left in the folder. one is the js that comes with the actual download, one is the modded version that sort works with pdfs. i dont know which is which anymore but 1 of them is setup for the pdf hack.
www.virtualnorthland.com/tb_pdf.rar
plus if they do not have the pdf brouser plugin then it will not work no matter what brouser is being used.