I needed to have phonetics symbols in the actual names of pdf files. What I used was the character palette (this is 10.3.9). You turn it on via system preferences: international: input menu. Check "character palette," "U.S.," and "show input menu in menu bar."
Then click on the flag in the menu bar, select "unicode" in the upper left "view" menu and locate the symbol you need. This part took me awhile because I first clicked on "IPA Extensions" to find the character, and it wasn't there. I wound up searching the entire unicode table and finally found the two I needed.
When you have the character you need, select the spot where you want the character to go, then in the character palette select a font (it will give you a choice), then click "insert" (or it may say "insert with font": not sure of the difference).
For example, I was able to put ð in many file names. After doing it once, you can copy and paste it into other file names.
Since this was for a CD to be included with a book, what really thrilled me was that I was able to burn a CD, open up the directory on both a Mac and a PC, and the symbols remained intact!