how do to reduce a file size? Without loss quality for large format to print

That's 30 000 pixels long side. It should be workable on a decent system - but nothing requires that kind of resolution (except a very very few special situations. This isn't one of them).

100 ppi is good enough here. That's still 10 000 pixels, a high resolution file by any standard.

Why do you want a 6+foot by 8+foot print that has a 300DPI resolution.   One would not view ant image that size close where the human eye can resolve down to 300DPI.  You could resample the image down to have a 150DPI resolution. Some image detail will be lost however, your eyes will not see that because of the distance you will be viewing the image from.  The 150DPI image will be 1/4 the size of the 300DPI image. People in a waiting line do not pixel peep poster and billboards prints. Go and look at some..... A 100DPI print  may even be good and be 1/9 the size as the 300dip image