If you are doing contracting or have a small business, you want to have some inexpensive tools to help you do your paperwork, such as offers and invoices, in an efficient and professional manner.
A good choice for Linux users would be the free and open source package Kraft.
Kraft is designed as a simple, yet comprehensive document creation and management software that includes customer management, offer and invoice creation in PDF format, templates, calculation, and material management. The software includes the functions to create copies of existing documents as starting points for similar ones, and to create follow-up documents, such as invoices from offer documents.
Other utilities include templates of items that can be included in documents. Such templates can include calculation modules for hourly or fixed pricing as well as material cost. The documents can be printed in customizable PDF format. Copies of the PDF files are automatically archived when the file is printed for the customer.
For more information see the Kraft project web site.