One of the great feature that Sony implemented on their PS1 emulator on the PSP is the ability to read manuals when pausing the game. This is accessed when pressing the Home button. A menu will pop up allowing various configurations such as the screen mode and the emulated disc-loading speed. Also available is 'Software Manual', accessible if a document exists on your PS1's game folder. If you've decided to make your own PS1 eboot.pbp files, you can easily create your own manuals or download them from this very useful site. In either case you will have to place the DOCUMENT.DAT file into the folder that contains the relevant PS1 eboot.pbp file.
Personally I can't be bothered by manuals, so I create text files that contains FAQs or maps (in PNG file format). This are usually for difficult PS1 games that I've not played, which tends to be plenty of obscure RPG titles. In any case all I've to do is use something like IceTea and generate the files through the program. You will need to assign each document file to a PS1 ID in order for the emulator to recognise the file to the game. For example the Disc ID for the US NTSC version of Dragon Quest VII is SLUS-01206. You will need that ID when generating the document. Once successful, simply copy the generated DOCUMENT.DAT file into the same folder as your eboot.pbp file.