Yes, that works (at least on device, not on emulator where pdfviewer is missing).
I used a renamed pdf-Dokument (test.pdf):
Code:
OpenBook ("/mnt/ext1/test.pdf", NULL, 0);
But for original "User_Guide_PocketBook_603(DE).pdf":
Code:
OpenBook ("/mnt/ext1/User_Guide_PocketBook_603(DE).pdf", NULL, 0);
did not work. Possibly there is a problem with length of filenames.