Thanks for that. I could have sworn I tried that shell script line but I must have had something different. I deleted what I was working on this morning so no way to tell what I was doing wrong.
One advantage of a script, aside from not having to upload documents to Dropbox, is you can open multiple documents at the same time. Here are two versions of what I had in mind. The first uses the Applescript path such as "Macintosh HD:Users:badunit:Desktop:grant.png" The second uses the POSIX path like your script "/Users/badunit/Desktop/grant.png". Both allow a range of cells to be selected and their files opened in one call to the script. Invalid paths are skipped. I do wonder how many it can handle before it misbehaves. I've only tried it on three at a time. A limit could be implemented if necessary.
Anyway, if a script invoked by a keyboard shortcut would be a usable solution, the OP has a few to choose from.
tell application "Numbers" to tell front document
tell active sheet
tell (first table whose class of selection range is range)
if (number of cells in selection range) > 5 then display dialog "Open more than five documents?"
repeat with c in (get selection range)'s cells
set filepath to (value of c)
try
tell application "Finder" to open file filepath
end try
end repeat
end tell
end tell
end tell
tell application "Numbers" to tell front document
tell active sheet
tell (first table whose class of selection range is range)
if (number of cells in selection range) > 5 then display dialog "Open more than five documents?"
repeat with c in (get selection range)'s cells
set filepath to (value of c)
try
do shell script "open " & quoted form of filepath
end try
end repeat
end tell
end tell
end tell