Junk characters occurs when exporting pdf from InDesign CS4.

I have received some art files which created in Illustrator CS5. I have placed the Art files in InDesign CS4 and export to pdf while I open the pdf file in Acrobat Pro 7 some Junk characters are appearing whereas I open the same pdf file in Acrobat Pro 9 it looks fine. How it shown the junk characters in Acrobat Pro 7 only. Can anyone let me know why this is happened in Acrobat Pro 7 only?