how to compare two files in textedit

Do not believe everything you read on the Internet. TextEdit can open both files separately and you can eyeball the differences in both document windows, but it is not a file comparator by design.

With plain text files, and if you have Apple's Xcode installed, you can launch its FileMerge tool in the Terminal as:

opendiff filemarks.txt pdfmarks.txt

with this visual result:

and another approach in the Terminal to compare two files is using the UNIX diff(1) command:

As for @VikingOSX's second suggestion using the UNIX diff option, I would suggest adding " --color=always " immediately after "diff " and before the file path names in order to have the output shown in color which can make it easier to see the differences with items in red being from the first listed file and those in green from the second file.

Since @VikingOSX's command uses the "more" command so you can pause the output, I am modifying it to use "less" instead since I know it can preserve the color output from the "diff" command:

diff  --color=always  filemarks.txt   pdfmarks.txt  |  less  -r

This will do basically the exact same thing as the second command @VikingOSX showed, but will show the output in color. I personally prefer to see the output in color as it just makes it so much easier to quickly see the differences.