Digitalizing documents is evaluated as a key value-added factor in greening maritime transportation. Covid-19 accelerates the need for digitalizing shipping documents. However, very few studies analyze digitalization in maritime transportation from the environmental aspect, which is an important topic considering the global emphasis on decarbonization. The methods to measure document-related emissions are mainly conceptual guidelines. Therefore, this research aims to develop an estimation framework with concrete methods to quantify the greenhouse gas reductions from digitalizing shipping documents. Taking Singapore and China as examples, this study compares blockchain with centralized systems and an integrated platform with separate platforms. The major results are 1) digitalization can reduce over 99% of document-related emissions; 2) passenger road transport to transfer documents is the largest emission source in paper systems constituting over 90% of emissions per shipping event; 3) blockchain and centralized systems have similar effects on emission reductions; so do integrated and separated platforms.