My favorite kind of software products are the ones that very clearly make life simpler. Being able to legally sign a document by clicking a button in an email and squiggling my mouse to make my signature is definitely one of those things.
You can provide that to your users with HelloSign! You can set up your documents there (it supports all the formats you'd need, like PDF, Microsoft Word, Powerpoint, etc) and start collecting the signatures you need very easily. Set up templates of your commonly used documents. Make sure your branding is present during the signing process. Get notifications when documents are reviewed and signed.
There are a bunch more killer features you should be aware of. For example, like I mentioned, you can sign documents without ever leaving your email with their Chrome browser extension for Gmail. Same with Google Docs and Salesforce!
Perhaps most importantly, you can use HelloSign right from your own interface through their API. That's great for all us developers interested in building seamless useful experiences right in our own products. You can embed documents directly on your website with just a few lines of code!