Attaching Files to Projects

I own a small, private construction business and am using Zoho Invoice to organize and budget my various projects. When I go to a new project sight and discuss various estimates, I write out a basic project invoice and description on paper. I like these because they are quick and I am used to them. In my office, however, I would love to keep them organized electronically. I use them to later build a project in Zoho Invoice. It would be SO nice if once finished, I could simply attach the scanned document (pdf) to the project summary page. This would help me so much in keeping organized! I know you can attach documents when you send out emails, but I would love an area to attach documents permanently to the project summary page.