An idea to suggest that this is just a sample of the full text

apps/files_sharing/css/public.css

@@ -29,6 +29,7 @@
 
 #imgframe .text-preview {
 	display: inline-block;
+	position: relative;
 	text-align: left;
 	white-space: pre-wrap;
 	overflow-y: auto;
@@ -41,6 +42,16 @@
 	-moz-user-select: none;
 	-ms-user-select: none;
 	user-select: none;
+	z-index: 1;
+}
+
+#imgframe .watermark {
+	color: #E3E3E3;
+	font-size: 75pt;
+	position: absolute;
+	width: 80%;
+	top: 100px;
+	z-index: 0;
 }
 
 /* fix multiselect bar offset on shared page */

apps/files_sharing/js/public.js

@@ -118,7 +118,7 @@ OCA.Sharing.PublicApp = {
 				url: url,
 				headers: {Range: "bytes=0-1000"}
 			}).then(function (data) {
-				var textDiv = $('<span/>').addClass('text-preview');
+				var textDiv = $('<div/>').addClass('text-preview');
 				textDiv.text(data);
 				textDiv.appendTo('#imgframe');
 				var divHeight = textDiv.height();
@@ -129,6 +129,9 @@ OCA.Sharing.PublicApp = {
 				if (divHeight > previewHeight) {
 					textDiv.height(previewHeight);
 				}
+				var watermark = $('<div/>').addClass('watermark');
+				watermark.text('SAMPLE');
+				watermark.appendTo('#imgframe');
 			});
 		} else if (previewSupported === 'true' ||
 			mimetype.substr(0, mimetype.indexOf('/')) === 'image' &&