Install gem

 ~/src/rails $ gem install pdf-writer
 Successfully installed rubyforge-2.0.4
 Successfully installed hoe-2.6.0
 Successfully installed color-1.4.1
 Successfully installed transaction-simple-1.4.0
 Successfully installed pdf-writer-1.1.8
 5 gems installed
 Installing ri documentation for rubyforge-2.0.4...
 Installing ri documentation for hoe-2.6.0...
 Installing ri documentation for color-1.4.1...
 Installing ri documentation for transaction-simple-1.4.0...
 Installing ri documentation for pdf-writer-1.1.8...
 Installing RDoc documentation for rubyforge-2.0.4...
 Installing RDoc documentation for hoe-2.6.0...
 Installing RDoc documentation for color-1.4.1...
 Installing RDoc documentation for transaction-simple-1.4.0...
 Installing RDoc documentation for pdf-writer-1.1.8...

Create rails project

This is rails 2.3.5

 ~/src/rails $ rails test
  create
  create  app/controllers
  create  app/helpers
  create  app/models
  create  app/views/layouts
  create  config/environments
  create  config/initializers
  create  config/locales
  create  db
  create  doc
  create  lib
  create  lib/tasks
  create  log
  create  public/images
  create  public/javascripts
  create  public/stylesheets
  create  script/performance
  create  test/fixtures
  create  test/functional
  create  test/integration
  create  test/performance
  create  test/unit
  create  vendor
  create  vendor/plugins
  create  tmp/sessions
  create  tmp/sockets
  create  tmp/cache
  create  tmp/pids
  create  Rakefile
  create  README
  create  app/controllers/application_controller.rb
  create  app/helpers/application_helper.rb
  create  config/database.yml
  create  config/routes.rb
  create  config/locales/en.yml
  create  db/seeds.rb
  create  config/initializers/backtrace_silencers.rb
  create  config/initializers/inflections.rb
  create  config/initializers/mime_types.rb
  create  config/initializers/new_rails_defaults.rb
  create  config/initializers/session_store.rb
  create  config/environment.rb
  create  config/boot.rb
  create  config/environments/production.rb
  create  config/environments/development.rb
  create  config/environments/test.rb
  create  script/about
  create  script/console
  create  script/dbconsole
  create  script/destroy
  create  script/generate
  create  script/runner
  create  script/server
  create  script/plugin
  create  script/performance/benchmarker
  create  script/performance/profiler
  create  test/test_helper.rb
  create  test/performance/browsing_test.rb
  create  public/404.html
  create  public/422.html
  create  public/500.html
  create  public/index.html
  create  public/favicon.ico
  create  public/robots.txt
  create  public/images/rails.png
  create  public/javascripts/prototype.js
  create  public/javascripts/effects.js
  create  public/javascripts/dragdrop.js
  create  public/javascripts/controls.js
  create  public/javascripts/application.js
  create  doc/README_FOR_APP
  create  log/server.log
  create  log/production.log
  create  log/development.log
  create  log/test.log
~/src/rails $

Config rails

Edit config/environment.rb and add

config.gem "pdf-writer", :version => "1.1.8"

Test

 ~/src/rails/test $ script/server
 => Booting Mongrel
 => Rails 2.3.5 application starting on http://0.0.0.0:3000
 no such file to load -- pdf-writer
 /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
 /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
 /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:156:in `require'
 /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:521:in `new_constants_in'
 /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:156:in `require'
 /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/rails/gem_dependency.rb:208:in `load'
 /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/initializer.rb:307:in `load_gems'
 /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/initializer.rb:307:in `each'
 /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/initializer.rb:307:in `load_gems'
 /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/initializer.rb:164:in `process'
 /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/initializer.rb:113:in `send'
 /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/initializer.rb:113:in `run'
 /Users/chris/src/rails/test/config/environment.rb:9
 /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
 /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
 /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:156:in `require'
 /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:521:in `new_constants_in'
 /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:156:in `require'
 /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/commands/server.rb:84
 /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
 /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
 script/server:3
 no such file to load -- pdf-writer
 /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
 /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
 /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:156:in `require'
 /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:521:in `new_constants_in'
 /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:156:in `require'
 /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/rails/gem_dependency.rb:208:in `load'
 /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/initializer.rb:307:in `load_gems'
 /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/initializer.rb:307:in `each'
 /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/initializer.rb:307:in `load_gems'
 /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/initializer.rb:169:in `process'
 /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/initializer.rb:113:in `send'
 /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/initializer.rb:113:in `run'
 /Users/chris/src/rails/test/config/environment.rb:9
 /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
 /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
 /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:156:in `require'
 /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:521:in `new_constants_in'
 /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/dependencies.rb:156:in `require'
 /opt/local/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/commands/server.rb:84
 /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
 /opt/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
 script/server:3
 Missing these required gems:
   pdf-writer  = 1.1.8

 You're running:
   ruby 1.8.7.174 at /opt/local/bin/ruby
   rubygems 1.3.5 at /Users/chris/.gem/ruby/1.8, /opt/local/lib/ruby/gems/1.8

 Run `rake gems:install` to install the missing gems.

Check that gem is present

 ~/src/rails/test $ gem list pdf-writer

 *** LOCAL GEMS ***

 pdf-writer (1.1.8)

Next step?

No idea