Add support to find PDF files in the system.

[akhildabral]

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Add a feature to be able to parse file system and find all the PDF files. This will make it easy to find all the hidden files (that one downloads and forget for ages). An example of it is Google's Picasa that searches for all the image files and gives a unified environment to access them.

[oguzhaninan]

Great idea. I will add it in the new version.