The ownership and or permissions/privileges on the document or file may need
to be unlocked or changed. There is a way to see the basics of this if you select
the file (click once) to highlight, then use Get Info from Finder menu (command + i)
You may have to see if the item has a padlock or who owns permissions, & change
them on just that file. Since other things may be involved, that I don't know about, it
would be up to you. Depending on the source of the file, and type, there may be a
few other items to consider. Some created files are limited by their author; one such
software may be adobe where a user can secure the document or content of a PDF.
When that happens, your options could be limited.
Also, external drive may be of an incorrect format for use with the Mac, & also window PC.
The lack of details can be create difficulty in communication. At 1:10AM tiredness is, too.
Good luck & happy computing! 🙂