The PDF file says what font to use. Adobe Reader doesn't make that decision, there's no setting controlling it.
Now, if what you see on screen has changed there are some possibilities.
1. You've made some other change to how PDFs are displayed. For example "font smoothing" doesn't change the font, but affects how clear they seem to be.
2. The fonts are not embedded in the PDF, so Adobe Reader has to substitute. Perhaps the font used for substitution has gone away.
Thank you so much for your help. I found ."font smoothing" under preferences and made a couple of changes - to lcd screen & local font. The font is clearer and looks like it originally did.
Again, thanks for taking the time to help me.