Add check that PDF file can be opened and is not malformed

This checks that the PDF file is not malformed in some way (e.g. by
missing the EOF marker). We check this by trying to get the page count
of the letter which will be needed to display the preview of the letter.

app/main/views/uploads.py

@@ -1,7 +1,16 @@
 import uuid
 from io import BytesIO
 
-from flask import flash, redirect, render_template, request, url_for
+from flask import (
+    current_app,
+    flash,
+    redirect,
+    render_template,
+    request,
+    url_for,
+)
+from notifications_utils.pdf import pdf_page_count
+from PyPDF2.utils import PdfReadError
 
 from app import current_service
 from app.extensions import antivirus_client
@@ -28,12 +37,16 @@ def upload_letter(service_id):
 
         virus_free = antivirus_client.scan(BytesIO(pdf_file_bytes))
         if not virus_free:
-            flash('Your file has failed the virus check', 'dangerous')
-            return render_template('views/uploads/choose-file.html', form=form), 400
+            return invalid_upload_error('Your file has failed the virus check')
 
         if len(pdf_file_bytes) > MAX_FILE_UPLOAD_SIZE:
-            flash('Your file must be smaller than 2MB', 'dangerous')
-            return render_template('views/uploads/choose-file.html', form=form), 400
+            return invalid_upload_error('Your file must be smaller than 2MB')
+
+        try:
+            pdf_page_count(BytesIO(pdf_file_bytes))
+        except PdfReadError:
+            current_app.logger.info('Invalid PDF uploaded for service_id: {}'.format(service_id))
+            return invalid_upload_error('Your file must be a valid PDF')
 
         upload_id = uuid.uuid4()
 
@@ -49,6 +62,11 @@ def upload_letter(service_id):
     return render_template('views/uploads/choose-file.html', form=form)
 
 
+def invalid_upload_error(message):
+    flash(message, 'dangerous')
+    return render_template('views/uploads/choose-file.html', form=PDFUploadForm()), 400
+
+
 @main.route("/services/<service_id>/preview-letter/<file_id>")
 @user_has_permissions('send_messages')
 def uploaded_letter_preview(service_id, file_id):