Is it possible to select an attachment in mails that he will send each time( like general conditions) ?
No, but you can set default contents in Notes field.
yes i know, but then you have no control about the layout. And i always print it now on the backside of the invoice( hole page)
under Settings you have Email Templates where you can create a default template for your sales invoice with the general conditions.
you can create the template in html too.
but then you have a hole page of text, its easier if you had an attachment
ability to attach files to email is still in the ideas category. you will have to wait till the same is implemented. no ETA.