Trying to use Libre on mac 10.13.2. When I save documents they're saved as 'Unix executable' files which I cannot then open on pcs.

If I save a document it’s save as an unusable ‘Unix executable’ file. I fail to open these files on pcs. If I try to open this file on my mac it will also fail unless open from inside Libre.

what is the filename extension of the files that you have seen?

I think Unique is on the right track, MacOS doesn’t use file extensions like Windows. Try to save the file with the extension in the name like document.odt.
Can’t hurt to try. Note:if long names still don’t work, try the classic DOS naming format of 8 characters followed by a dot followed by the 3 characters of the extension.