PDFs are final files - nobody should be editing them.
However, you set the Font to Subet at 0% (meaning every glyph will be embedded) and they can then insert any character from that font set when editing.
They will need acrobat pro to edit the text - but only minor edits may be done using the Advanced menu in acrobat pro
There are a few ways to add text - perhaps this will help http://www.creativepro.com/article/acrobat-how-add-text-pdfs
If you have Acrobat Pro, you can add form fields in that application. Forms > Add or Edit Fields...
You have minor control over the formatting using the form field properties.
I have used this to allow customers to add their own contact information to advertising designs.
You can save the PDF with 'extended features' for Acrobat Reader that will allow the end user/client to use Acrobat Reader to edit the text fields and save the document. Advanced > Extend Features in Acrobat Reader...
NOTE: this is a Save As and locks the form for editing. Make it the last step. It will not allow them to change text that is not set up as a form field.