On a Plesk server, the mod_deflate module is installed by default, however it may be disabled in the Apache configuration file. To enable the module edit the Apache configuration file
vi /etc/httpd/conf/httpd.conf
Search for the line
#LoadModule deflate_module modules/mod_deflate.so
and uncomment it i.e. remove the ‘#’ mark
LoadModule deflate_module modules/mod_deflate.so
Save the file and restart the httpd service
service httpd restart
Now, create a .conf file under the /etc/httpd/conf.d/ directory since Apache reads all the .conf files from that directory on a Plesk server
vi /etc/httpd/conf.d/deflate.conf
and place the following code in it
<Location />
SetOutputFilter DEFLATE
SetEnvIfNoCase Request_URI .(?:gif|jpe?g|png)$ no-gzip dont-vary
</Location>
Save the file and restart the httpd service. The compression code will compress all the files except the .gif, .jpe, .jpeg and .png files. To test the compression, use the tool
http://www.whatsmyip.org/mod_gzip_test/
To enable compression for a specific directory or domain, specify the directory path in the <Location> directive in deflate.conf and restart the Apache server.