Is resetting (that is reinstall win but keep personal files) good enough to remove all possible malwares and viruses if exist, or better backup everything, wipe and do clean install.
Performing a Reset within Windows performs the same steps that reinstalling Windows will perform. However, in either case, you are going to have to choose NOT to keep your personal files otherwise you would be just transferring the malware to the new installation. If you don't have an ISO for the version of Windows 10 you are running you can download one here.
There is also an option to reset and NOT keeping files. Is it exactly like choosing reinstall?
Yes
After downloading and creating a USB, wheat needs to be done to reclaim the license for the new installation?
You have indicated Windows 10 is activated and has a digital entitlement. If you were to reinstall Windows 10 it would automatically activate. You can reinstall Windows 10, from within Windows, by mounting the ISO for the version of Windows 10 you wish to install. Be sure you select the option NOT to keep any personal files otherwise you run the risk of transferring the malware to the new installation.
Source: Windows Setup Process
Is it possible a malware is actually on the Documents folder, and when copying it back will cause the fresh installation be actually infected again?
Yes; This is the reason I specifically indicated you should NOT keep any personal files. However, you are the one who believes you are infected with malware, despite security software, saying you are not actually infected. However, you are the best person, to determine what risks you are willing to accept.
