Updated copy on the page.

Added unit tests.
Added error when file is too big.

app/main/views/platform_admin.py

@@ -280,6 +280,14 @@ def letter_validation_preview(from_platform_admin):
             ), 400
 
         try:
+            if len(pdf_file.read()) > (2 * 1024 * 1024):
+                return render_template(
+                    view_location,
+                    form=form,
+                    message="File must be less than 2MB",
+                    pages=pages, result=result
+                ), 400
+            pdf_file.seek(0)
             response = validate_letter(pdf_file)
             response.raise_for_status()
             if response.status_code == 200:

app/templates/views/letter-validation-preview.html

@@ -5,6 +5,11 @@
 {% from "components/page-footer.html" import page_footer %}
 
 {% from "components/form.html" import form_wrapper %}
+
+{% block service_page_title %}
+  Letter validation preview
+{% endblock %}
+
 {% block maincolumn_content %}
 
 <div class="grid-row">
@@ -15,14 +20,29 @@
       {{ banner(message, 'dangerous')}}
     {% endif %}
 
-    <h1 class="heading-large">Letter Validation Preview</h1>
+      <h1 class="heading-large">Letter validation preview</h1>
     <div class="bottom-gutter">
      {{ file_upload(
           form.file,
-          action = url_for('main.services_letter_validation_preview', service_id=current_service.id),
-          button_text='Upload a PDF document',
+          action = url_for('main.service_letter_validation_preview', service_id=current_service.id),
+          button_text='Choose a file',
       )}}
     </div>
+      {%if not pages %}
+        <div>
+        <h1 class="heading-medium">Check your file meets the letter specification </h1>
+        <p>Your file must be:
+        <li>a PDF</li>
+        <li>no more than 10 pages long</li>
+        <li>less than 2 MB</li>
+        </p>
+        <p>The content of your letter mut appear inside the printable area.</p>
+        <p>
+        <a href="https://docs.notifications.service.gov.uk/documentation/images/notify-pdf-letter-spec-v2.3.pdf">
+            Download the letter specification</a> for more information.
+        </p>
+        </div>
+    {% endif%}
   </div>
   <div class="column-whole template-container" >