Paper invoice is well established and understood by everybody (your solution may not be).
Paper invoice doesn't require owning a 3G-connected smartphone with active app store account backed by email and credit card and passwords and recovery codes (your solution may require such).
Access to paper invoice cannot be unilaterally revoked by the invoice issuer (might not be the case in your solution).
Paper invoice will just be there for years and years, and can be presented when required by IRS audit in 20 years (might not be the case in your solution).
There are organizations that email me the invoices as attached PDFs. This is fine, as it can be easily turned into a paper invoice. There are organizations that email me the link to invoices. This is not fine, as the link may expire, so I have to remember to download the invoice shortly. There are organizations that require me to download an app of their choice to view their invoice, and I absolutely refuse to deal with them. My phone is there for my convenience, not their convenience, and I choose the apps that run on my phone, while they don't have any say in the matter.