Sudden change in fonts

  Hi! I have trouble with finidng out the missing font of my pdfs. I could view those files really well before, but all of a sudden it says a font is missing and from then some of the words in all the pdfs that I have had their fonts changed. I accidentally closed the 'note' telling me which font I was missing and now I couldn't seem to be able to find it. How would I be able to do that? And how can I install the missing font?

  Thank you.