Helvetica Neue is not open source. It is a commercial typeface owned by Monotype Imaging. Downloading it without a license is software piracy. If you use a pirated font in a commercial client project (a logo, a billboard, a published book), you can be sued for thousands of dollars. Font foundries have automated bots that scan PDFs and websites for unlicensed fonts.
The "LT Std" version is actually thinner than the default Mac version. Designers prefer it for luxury branding because it feels more elegant on paper. download helvetica neue lt std font family