Htmlpdfに直リンクする（ダウンロードしない）方法

Add to app/models/attachment.rb, somewhere next to def thumbnailable:

def pdf?
!!(self.filename =~ /.(pdf)$/i)
end

Then modify app/controllers/attachments_controller.rb:

Change the line:

:disposition => (@attachment.image? ? 'inline' : 'attachment')

to

:disposition => (@attachment.image? || @attachment.pdf? ? 'inline' : 'attachment')