Remove pdf metadata

In order to remove metadata from PDF documents we will use the
exiftool_vendored gem.

The following line:
  Exiftool.new(attachment_path, "-overwrite_original -all:all=")
Overwrites the original file with another file without metadata.

So far this is the best solution we have found to perform this
metadata deletion.

When using Exiftool an exception is thrown, so we added a rescue
to handle it. Here is a task created where this problem is discussed
in issue 28 in the https://github.com/exiftool-rb/exiftool.rb/ repository.
We'll wait to see if this will be fixed in future versions.

app/models/document.rb

@@ -9,6 +9,8 @@ class Document < ApplicationRecord
   validates :documentable_id, presence: true,         if: -> { persisted? }
   validates :documentable_type, presence: true,       if: -> { persisted? }
 
+  before_save :remove_metadata
+
   scope :admin, -> { where(admin: true) }
 
   def self.humanized_accepted_content_types
@@ -36,4 +38,13 @@ class Document < ApplicationRecord
     def documentable_class
       association_class
     end
+
+    def remove_metadata
+      return unless attachment.attached?
+
+      attachment_path = ActiveStorage::Blob.service.path_for(attachment.key)
+      Exiftool.new(attachment_path, "-all:all=")
+    rescue Exiftool::ExiftoolNotInstalled, Exiftool::NoSuchFile
+      nil
+    end
 end