Is there any easy way of doing it?
yes, there is...
- go to export map
- select the filetype to png
- in options, change to "format" tab
- colour mode must be in 24 bits
- in "background colour" select "no color"
- in "transparent colour" click "other colours" and change all values to 240 (arcmap changes the background colour in export from ffffff (pure white) to f0f0f0 (red 240 green 240 blue 240) but keeps the white on all elements that are set to white in the symbology