problem to search a word in pdf file with "Document Viewer"?

I'm using Ubuntu 12.04 64 bit.

I downloaded a PDF file for my academic requirements. When I open this PDF with Document Viewer, it displays the contents of the file. However, when I start searching (with Ctrl+F) for a word in the file, it gives me "no word is found" for every attempt even though it is present the file?

How can I search for the word in the PDF?

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