I am using Redmine for internal issue tracking.

I wanted PDFs to be displayed in the browser. This required a quick-and-dirty hack, which I found here: https://www.redmine.org/boards/3/topics/26320

The essence of the hack:

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')