In order to convert tabs with spaces in Notepad++, go to Menu: Edit -> Blank Operations and select TAB to Spaces

Example 2:
If you want to convert each tab in the file with single spaces, you can make use of the Find and Replace option with Search Mode Extended as follows,
- Open File in Notepad++
- Press Control + F to Open Find and move to Replace Tab,
- Make sure to select Search Mode as Extended,
- Add, Find What: \t
- Add, Replace With: (a single space, or multiple if you want it that way!)
- Click on Replac All

Example 3:
If you want to replace TAB by space (4 spaces or more) as default permanent behavior, then you can do the changes in Preferences as follows,
- Go to Menu: Settings,
- Now select Preferences...,
- Select the Language tab,
- Now under tab settings check the Replace by space checkbox.

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