Some pdf have some weird font and it would be great if you could change the font to have a better readability.
You need a PDF editor like Acrobat for that and not every PDF can be edited and even when a font can be changed other issues i.e. with layout can arise.
PDF is a format for printing, so it focuses on defining exactly how pages should look and it is not easy at all to edit PDF files.
It is very different from HTML and ePub, for example.