pdf import missing characters

Hello, When importing pdf files into s note, the files are missing the characters that they contained originally. This happens with polaris as well. All of the words are present but mathmatical characters are replaced with blank boxes. When opened with adobe reader they are all there. I need to be able to import the pdf files for school. I have searched the internet for a resolution to the problem but have found none. Any help would be appreciated. Thank you.