Video is 72 pixels/inch also at TV news broadcast. If you use 300 pixels/inch in the timeline for stills it will be rendered to 72 pixels/inch.
Render time of 72 pixels/inch stills in project framesize will save a lot of render time. And don’t worry about clarity of the text. It will be OK.
If you have separate pixtures for the document details, exporting frame from the timeline is not necessary.
By the way, if you want to make a frame from the timeline from one particulary track and you have f.e. two tracks at that location you have to switch off the other track temporary.
Phocas Kroon