Problem with document streaming.

Hello all.......
I am facing a problem with streaming of a document. I have a site from where i can download PDF files (ranging from 500 Kb to 2 GBs).

When i try to open after downloading PDF files of more than 10Mb, i get error message "There was an error opening this document. The file is damaged and could not be repaired." - (By acrobat reader :: tried with different versions).

When i checked its file size, i found that document received is of improper size. Everytime i download the same document, i get different file size.

I am reading data in chunks (of 10Kb).
I dont know why it is behaving in such a manner.... Please help

Thanks
MMH