Notepad++ is free software (as in both "free speech" and "free beer") distributed under GPL license. Please refer to the license.txt file in the distributions for precise terms and conditions.
A number ofdistributors provide Notepad++ on their own sites for easy access to freeware and shareware resources. This is obviously a good thing for the worldwide user community.
Unfortunately, some have addd componnents to their redistribution of Notepad++ so as to increase their advertisemment income. This practice is not endorsed by the project even remotely, and is a blatant breach of the terms of licence just mentioned. The offenders are mostly out of reach of legal action.
In order for users to check whether the archive and binaries downloaded from sources other than http://notepad-plus-plus.org/ , there is a forum whose sole purpose is to post SHA-1 signatures for the distributed packages: the SHA-1 digests of binary packages forum.
How to check a file through its SHA-1 digest ?
You can obtain free SHA-1 digests online by uploading the file to verify. http://hash.online-convert.com/sha1-generator is such a page. Pleas check their terms - this page does not endorse the site, mentioning it only for the purpose of referencing.
There are tools to calculate SHA-1 digests offline, like jdigest (choose the latest executable jar file). While this does not require any installation, your system must know how to execute Java .jar files.