Custom apps?

[timparsons]

To what extent can apps be created? Are there guidelines?

I'm trying to see if I can create an app that would fill in some form data and then be able to upload it to a flyer. For example, a flyer with a space for a custom address on the bottom. Someone could add in an address, the flyer would be updated with the new address and then it could be saved as a .pdf.

[#2]

@timparsons Here is a fairly exhaustive page that links to all of the development frameworks available to you in SharePoint (depending on version of course). 

https://docs.microsoft.com/en-us/sharepoint/dev/general-development/sharepoint-development-overview