NNR gave me his Kobo Vox ereader to use. The epubs available for download from instapaper do not open in the Kobo's built-in library, but open in FBReader which I installed. Converting a mobi downloaded from instapaper to epub using Calibre could make that file recognizable by Kobo's library app. But with any method, the Feynman lectures' equations don't render correctly in epub. Printing to pdf using PDF995 and reading with Adobe Reader works nicely.