Every file uploaded to Resources is renamed into a safe form as it is stored, and the name you see in the folder afterwards is the renamed one. Only the letters A to Z, the digits, the full stop, the underscore and the hyphen survive. Everything else is replaced.
Plan Set - Rev2.pdf becomes Plan_Set_-_Rev2.pdf. A name with accents in it loses them: façade détail.pdf becomes fa_ade_d_tail.pdf. Several unwanted characters in a row collapse into a single underscore, so Site Plan (final) v2.dwg becomes Site_Plan_final_v2.dwg.
A name written entirely in a non-Latin script has nothing to keep and comes out as an underscore and the extension. Rename those on your computer first if the name carries meaning you need.
Folders you create are treated the same way.
Very long names lose their beginning
The stored name is capped at 200 characters, and it is the last 200 that are kept. That is the opposite of what most people expect: the extension and the end of the name survive, and the front is cut away. A name long enough to reach the cap is worth shortening yourself, so that you choose which part goes.
Why this matters beyond appearances
Resources decides whether an upload is a new file or a new version of an existing one by comparing names within the same folder, and it compares the renamed forms. So two names you think of as different can collide.
Plan Set.pdf and Plan_Set.pdf both become Plan_Set.pdf. Uploading the second turns it into version 2 of the first rather than adding a second file, and the folder now holds one entry where you expected two. Nothing is lost: File versions in Resources covers how to read the versions and get the earlier one back.
You cannot correct it afterwards
There is no rename in Resources. To fix a name, rename the file on your computer and upload it again, remembering that uploading it under the old name will version the existing entry rather than replace it. See You cannot rename or move a file in Resources.