It's really broad to ask about whether a document is considered "secure". Something that is "secure" enough for me, sending a love poem to my girlfriend, is not "secure" enough for Obama, sending a love poem to Putin.
As Alexander O'Mara mentioned in a comment, there are different ways of setting a password on a file.
Excel gives you several ways to protect a workbook.
You can require a password to open it, a password to change data, and
a password for changing the file's structure—adding, deleting, or
You can also set a password in Backstage view that encrypts your
Remember, though, that this type of protection doesn't always encrypt
your data - only the encrypting password created in Backstage view
does that. Users can still use third-party tools to read data that
(from here )
So it seems that simply password protecting an excel file would not be very "secure", as someone might still be able to view the data with a third party tool.
That said, assuming you want edit protection, it again depends what you later do with the password, or how you would transmit it to the person later using it.