Sadly pdf is not a useful format to put in readers. Not sure whether they cover the exact same ground but these have more file variety:

Gutenberg http://www.gutenberg.org/ebooks/author/1308

Standard ebooks https://standardebooks.org/ebooks/seneca/

If you have time you should check this one out: https://en.wikipedia.org/wiki/Epistulae_Morales_ad_Lucilium

Bit cringey when Seneca's mansplaining to a mother struggling with son's death why she shouldn't be so gloomy.