How can I remove all spaces from file in Notepad++?

To delete all spaces in the file, replace ' +' with '' (quotes only for demonstration, please remove them). You need to have the checkbox "Regular expression" checked.

To remove all spaces and tabs, replace '[ \t]+' with '' (remove quotes).

This works for big files, too, where the solution of @Learner will be tiresome.

No need for regular expressions. In Find what add a whitespace character and then replace with nothing.