Your web files will be viewed by numerous users who use a wide variety of operating systems (Mac, PC, and Linux for instance) and devices (desktops, tablets, and smartphones). Therefore, it is essential to avoid common "illegal" directory and filename characters.  Naming conventions are important in web folders as well as for downloadable files such as PDF, Word documents, and Excel spreadsheets.

Do not use any of these common illegal characters/symbols to avoid potential issues:

• # - Pound

• % - Percent

• & - Ampersand

• { - Left bracket

• } - Right bracket

• \ - Back slash

• < - Left angle bracket

• > - Right angle bracket

• * - Asterisk

• ? - Question mark

• / - Forward slash

•  - Blank spaces

• $ - Dollar sign

• ! - Exclamation point

• ' - Single quotes

• " - Double quotes

• : - Colon

• @ - At sign

• + - Plus sign

• ` - Backtick

• | - Pipe

• = - Equal sign

Reference: http://www.mtu.edu/umc/services/web/cms/characters-avoid/