let's assume you are trying to open "1.txt" on the root of sd card with the default browser.
1.launch the default browser;
2.key in "content://com.android.hmtlfileprovider/sdcard/1.txt" in the address box of the browser then go;
If 1.txt is not a ansi text file, change the encoding type in the browser to get a proper display.
then you can use the print to eink function of the default browser to read it on eink.