I have installed Adobe Reader DC ver. 2021.001.20145, strangely and randomly when viewing pdf documents, some pages appear fine and others with illegible characters, these are letters without any meaning.
Hope you are doing well and sorry for the trouble. As described, you are unable to read as some text appears with illegible characters.
Have you created this PDF file or have you got it from a different user? It looks to be the fonts not embedded in to the PDF file or the fonts are not present on your computer. Please go to Edit (Win) Adobe Acrobat (Mac) > Preference > Page Display > Under Rendering > Select 'Use local fonts' > Click OK and reboot the computer once.