I do not have PDF Exchange Viewer but I would imagine that "save as" will rename the file to whatever you want to call it while "save copy as" will create a new file called whatever you want to call it while leaving the original file as it was.
Xiaomi Mi Mix 2 review
See mcbess's iconic style animated for Mercedes-Benz
iPhone X news: Release date, price, new features & specs
Black Friday 2017 : date, sites participants & bonnes affaires