add help for pdf import

vue/Imports/Files/ImportFromFiles.vue

@@ -53,7 +53,14 @@
         </div>
         <div class="control">
           <form v-on:submit.prevent="start">
-            <div class="is-pulled-right has-text-right">
+            <div class="is-pulled-right has-text-right field is-horizontal">
+              <label class="label align-center" v-if="isPdf">
+                <abbr
+                  title="Poppler is recommended for PDF files with more than five pages"
+                >
+                  PDF import method
+                </abbr>
+              </label>
               <div class="field has-addons">
                 <div class="control" v-if="isPdf">
                   <div class="select">