You should know that the font can be changed only for a few file formats. For example, you can't change the phone in a PDF file through the eBook Readers. I have tried Moonlight Reader (known as the recommended ebook reader), but the Reduce Font worked only for ePUB, MOBI, TXT, DOC. No support was for PDF, which could be the case in this situation. Try using another application to view your ebooks and then change the font. Use App Store or Google Play to install other ebook readers.