Sadly the PRS-300 is dead from years (broken display) it was the better e-reader I ever had not thank to Sony but thanks to the alternative firmware "EbookApplications"!
The battery issue was that maybe the icon was present (I don't remember) but at least in the Kobo HD I have no alert and so if you not noticed it the battery become empty and you discovered it only when you tried to put your device... and when you recharged the device the Koreader settings were gone! The last page was not remember (it restated from page 1), the font size was wrong... all lost...
The battery problem was with Android too and it is for this that after Kobo Aura HD died (I don't say how it died I'm to shy to say it
) I'm using only the stock reader in the H20...
The RFT is really a simple format a sort of glorified TXT file with some mark up for write text in bold, in italic and so on... some version support embedded images too (I don't know if Kobo show images on RTF the old Sony with default reader cannot for example but the one in EbookApplications supported this), I cannot imagine what could make them invalid obviously WordPad and Openoffice open the files correctly and I could read until the end!
Tonight post one of the RTF files that are truncated on the Kobo.
Thank you for your help.