PalmOS text editors is a broadly based topic, because there are different things that can be considered text on Palm devices.
The design of PalmOS requires in file in RAM to be in PalmOS database form. A program is a PalmOS "resource" database. Databases can contain arbitrary content, and the OS uses the Creator and Type attributes of the database to know what programs use what data. As of PalmOS 4, PalmOS added support for VFS (Virtual File System), a high level interface for access to expansion cards. Expansion cards are formatted with the MS-DOS FAT file system, and seen as a disk by PalmOS and applications. Other types of files that are not Palm databases can be stored on cards, assuming applications that can use them are installed.
All Palm devices ship with a MemoPad? application. MemoPad? can be used to create and edit memos. Individual memos are records in the memo database. Earlier Palm devices had a limitation of 4K for memopad records. Newer devices increase that to 32K. Memopad can be considered a text editor, and there are editors for PalmOS specifically designed to supplement or replace the built in memopad application.
The defacto standard text file on PalmOS devices is the "doc" file. A doc file is a plain ASCII text file in Palm database format, compressed with a form of RLE compression to save space in RAM. Applications that view/edit doc files decompress them on the fly. The doc format was originated by Aportis, which no longer exists, but the format was reverse engineered and widely adopted.
A newer format that may replace doc files is the zText file. zText files are also plain ASCII text files, but use a Palm port of the Zlib library to provide gzip compatible compression.
If the Palm device has VFS support and an expansion card installed, actual ASCII text files may be stored on the card, and viewed and edited by programs designed to do so.