"You scanned pages, made them into one .pdf file and size of this file is 300 mb. But it has to be not more than 30-40 mb". :) 
I offer you to discuss compresion methons only for GNU/Linux systems, particularly because of my egoism :) (my OS is Linux Mint 10)

That's the only method I know how to compress:

1. I installed Ghostscript and pdftk (sudo apt-get install ghostscript pdftk);

2. Converted my .pdf (240 mb)  in .ps file 

(pdf2ps file1.pdf file2.ps) . .ps'll have more size, than your .pdf(I got 400mb).

3. Then I  converted this  .ps file again to  .pdf(ps2pdf file2.ps file3.pdf)

To sum up, I got  9mb .pdf file with the same quality as 240mb's.

If somebody can expain, how can it be or knows any other methods of compression, write in comments)