Trouble with add_filter

Hi everyone!

I’m writing my first plugin and I have to face with probably small problem. I have such a code:
add_filter( 'the_content', 'my_func' );
And it works, but when I put something like this:

if( is_page() )
{
  add_filter( 'the_content', 'my_func' );
}

It doesn’t work. The question is of course “why?”.

And BTW: I check if is it a static page or post by checking post_type in DB. Is there any better solution?

Thanks in advance for any help!