Backstory: I've been sending a bunch of PDFs to clients lately, product manuals and statements of work. Mostly I used to send them from indesign but now I send them from google docs due to simplicity and speed. There's room for something in between.
The Idea: Browser PDF - create a simple PDF in the browser
Deep Dive: Keeping this simple is key. Focusing on ingesting a google doc and exporting a styled PDF would add a lot of value for small businesses/agencies.
Validation: Professional PDFs are always valuable for any business, particularly small businesses that don't have time to put together an inDesign template.
Monetization: I know recurring is the golden ticket but charging per pdf project may be a good way to start this one. Looking at HelloSign, they have a 3 document limit per month for free. Small businesses love it but HelloSign probably hates it ;) If you had something that was $3-5 per pdf, you would be able to capture some revenue from these small companies.
Steps to Launch: I would target consulting agencies and build a "statement of work decorator." Users can upload a google doc and produce a pretty looking pdf from some premade templates.