i have a folder named "download" in public-Html folder. all downloadable files on my site, is in it. when i backup my site with Akeebabackup, this folder makes my backup file Biiiig. what should i do if i don't this folder being contained in my Akeebabackup file?
In akeeba backup component there is an option to exclude the files or folders which you don't want.
Go to the akeeba backup setting Files and Directories Exclusion, exclude the folder which you don't want and then run the backup.
And for more info you can read this: Exclude data from the backup
Here is a video example.