Last night I was trying to translate Dröge, E., Maghferat, P., Puschmann, C., Verbina, J., & Weller, K. (in press/2011). Konferenz-Tweets. Ein Ansatz zur Analyse der TwitterKommunikation bei wissenschaftlichen Konferenzen. Proceedings of the 12th International Symposium for Information Science (pp. n.n.). Boizenburg: Verlag Werner Hülsbusch. (pdf link to paper).
I thought The Google Doc Viewer would have this feature built in but alas no. The clumsy workaround is to drag and select the text (Ctrl+A grabs the Doc Viewer interface) before coping and pasting into Google Translate.
I then discovered that copying the pdf url into Google Translate does the job without the hassle of copy/paste (link to translated pdf).
[The caveat to all this is the original pdf must be accessible ie still contain the raw text rather than being an image]