You need to check out the Window>Info
There are two values
With Actual it refers to the amount of PPI the image has when placed at 100%
With effective - it refers to the amount of PPI the image has when scaled.
An image that is Actual 72 PPI when placed at 100% is 72 PPI
If you scale that image to 24% the Effective PPI is 300 PPI
72/24*100 = 300
If you place an image at 100% and the Actual PPI is 300
If you scale the image up - to say 150% then the Effective PPI is 200 PPI
So if I made 50%, then decided to go 85%, if I don't go bigger than 100%, quality won't be lower.
[Links and promo information removed by moderator]