Opening a PDF file in Illustrator usually guarantees a warning. A nice list of missing fonts… When designing you should keep to the 3 font rule: Use no more than 3 fonts for each project, depending on project size. This is to allow a consistency throughout the project. If you use too many fonts for the size of project you may make your audience feel lost in translation and wind up not getting the intended message.
In opening a file in Illustrator I now know which fonts I need to replace. I can then exchange the fonts with something similar, majority of the public will not even notice. But beware if you are working with a client who is in the printing industry they may notice and if that’s the case I would prepare them in advance that a few changes were necessary to finish the project.