I also like cool-reader. But run both 32 and 64 bit versions as "external applications". See,
http://murga-linux.com/puppy/viewtopic. ... 121#996121.
Despite originating as a "deb", they run under any Puppy with the proper architecture (32 vs. 64). As noted there, you will have to install a couple of libraries into each Puppy. IIRC, the libraries needed were libQtGui.so.4 and libQtCore.so.4. But to be certain your Puppy provides all necessary libraries, file-browse to /usr/bin/cr3 in the extracted folder, right-click it and from the popup menu select "ldd". Click the "Missing" Tab at the bottom of the GUI which will open.
For all but the latest Puppies, their Puppy Package Manager will offer the required missing libraries. But newer Puppies have "progressed" to Qt5s. For those, open your web-browser to
https://pkgs.org/ enter the missing lib in the Search Box. Download the lib from the distro version closest to that of which your Puppy is "binary compatible".
As I noted in the post referenced above, I created a pet in order to have a menu entry (and install the pet into each Puppy where a menu entry was desired).
The "Error while opening document %F" noted is a consequence of beginning its existence as an Ubuntu Applications. You'll find that /usr/share/applications/coolreader.desktop has in its Exec= the %F argument. Under Ubuntu that would enable coolreader to open a specific
File perhaps from a right-click menu. Puppy has its own way of enabling specific file-types to be opened via Right-Clicking files. Just delete the "%F from the argument and save the change. Remember to execute a SAVE to your SaveFile/Folder.