class Document < ActiveRecord::Base
  belongs_to :user
  has_attached_file :doc_pdf

  validates_attachment_content_type :doc_pdf, content_type: "application/pdf"
end


class Document < ActiveRecord::Base
  belongs_to :user
  has_attached_file :doc_pdf, styles: {:thumb => "100x100#",
      :small  => "150x150>",
      :medium => "200x200" }

  validates_attachment_content_type :doc_pdf, content_type: ['image/jpeg', 'image/png', 'image/gif', 'application/pdf']
end

--- 
+++ 
@@ -1,8 +1,7 @@
 class Document < ActiveRecord::Base
   belongs_to :user
-  has_attached_file :doc_pdf, styles: {:thumb => "100x100#",
-      :small  => "150x150>",
-      :medium => "200x200" }
+  has_attached_file :doc_pdf
 
-  validates_attachment_content_type :doc_pdf, content_type: ['image/jpeg', 'image/png', 'image/gif', 'application/pdf']
+  validates_attachment_content_type :doc_pdf, content_type: "application/pdf"
 end
+
Remove attachment styles and unnecessary content type
