If you make a transcription ("derivative work") of a public domain copy of the work, then you can do whatever you like with it, including selling it for profit.
In the case of the site's great .pdf files, there are two copyrights to consider: firstly, the copyright held by the original author/publisher, which generally expired long long ago, and secondly, the copyright of the derivative, digital work made when the text was scanned and converted into a pdf file.
It is this second copyright that is an issue here, so if you wish to distribute your work you will have to obtain permission from the copyright holder, namely Jeff.
If you think this is confusing, consider the case of music copyright, where you can have an original composer, an original publisher, a republished copy of the originally published score, a modern copy of the republished copy, transcriptions and arrangements to other instruments, modern issues of the original published score, copyrights and duties for the performance of the score, copyrights and duties for the recording of the score, copyrights and duties for sampling the recording of the score, and so on.
That said, I think your idea is a good one and I hope that Jeff has no objections. A plain text copy will create not only smaller files, but also files that print better, are easily searchable, and so on..