Why Do My Documents Show As Read-Only For Others?

I have created a spreadsheet and saved it to a network drive. I have not
applied any sort of protection but when others try to open the file it shows
as Read-only. This also happens in reverse order; when I open a file created
by a co-worker it shows as read-only for me. Why is this?

It's possible that it's in the settings of your network folder.

Try to see the settings of your folder.

Check 1) file property - if it's read only. 2) folder property-Other uses may have read only access