Hi,
I'm Dyari. Thanks for reaching out.
Could you confirm how the files were encrypted? Did you encrypt them by right-clicking the folder > Properties > Advanced > Encrypt contents to secure data?
If yes, this is EFS encryption. EFS uses an encryption certificate and private key stored in your original Windows user profile.
Before reinstalling Windows, did you export or back up your EFS certificate, usually as a .PFX file?
If you have the certificate backup, you can import it into your current Windows installation and should then be able to access the files.
If you do not have the original EFS certificate and the previous Windows installation was erased, unfortunately Windows cannot generate the old key again. Signing in with the same Microsoft account does not restore the EFS certificate.
The Microsoft Information Protection Viewer would not help if these files were encrypted using Windows EFS.
You can also right-click one of the affected files > Properties > Advanced > Details to check which certificate was used to encrypt it.
Please keep me posted.