Sharepoint - Hide/disable ribbon bar buttons with client-side code

here the answer:

<script type="text/javascript">
    function hideEdit() {
        var edit = document.getElementById("Ribbon.ListForm.Display.Manage.EditItem-Large");
        edit.style.display = "none";
    }       
    _spBodyOnLoadFunctionNames.push("hideEdit");
</script>

Or

Enable/Disable ribbon button based on current item value of a field

I was able to do this using pure CSS, but had to escape the "." As follows:

<style type="text/css">
#Ribbon\.ListForm\.Display\.Manage\.EditItem-Large  {display:none !important;}
</style>

Did you mean EditForm.aspx? Cause for Doc libaries there's no NewForm.aspx

To hide it using CSS:

#Ribbon a[id$="Cancel-Large"] {
    display: none !important;
}

To hide it using jQuery:

$("#Ribbon a[id$='Cancel-Large']").hide();

I would suggest using a combination of both and use remove() instead of hide(). You could add the jQuery script using JSLink or Script editor.