There's no single, readily available statistic that gives the exact percentage of *all* US college graduates who graduated from Penn State. To calculate this, you would need the total number of US college graduates each year and the number of Penn State graduates each year. Those numbers are tracked separately by different organizations and aren't usually compiled into a single percentage like that.
You can, however, find the number of Penn State graduates each year on Penn State's website and the total number of US college graduates from the National Center for Education Statistics (NCES). You could then calculate an approximation for any given year. But it won't be a precise, universally agreed-upon percentage.