Give software engineers a dark room, three computer screens, and a hoody to wear, and they're generally pretty content people. They don't ask for much, but they do ask that website users avoid certain characters when creating files. The reason is that many of these characters are used in the underlying code.
Here is a quick list of characters that you'll want to avoid:
blank spaces (Yes, they're not technically characters on a keyboard, but browsers make them look like this: sample%20file.pdf)
# pound
< left angle bracket
$ dollar sign
+ plus sign
% percent
> right angle bracket
! exclamation point
` backtick
& ampersand
* asterisk
‘ single quotes / apostrophe
| pipe
{ left bracket
? question mark
“ double quotes
= equal sign
} right bracket
/ forward slash
: colon
\ back slash
@ at sign
This list was adapted from: Michigan Tech