> Is there a way to open it with the pdf viewer in morphos?

MorphOS comes with 2 PDF viewers, the older Apdf and the newer VPDF. Apdf prompts a requester for password input when you try to open a PDF file with it that is password-protected from viewing, so this should work (haven't tried, though). Problem is that Apdf lacks compatibility with newer PDF standards so the pages may look wrong (missing transparency etc.). VPDF (which correctly displays newer PDF standards) doesn't support password-protected PDF files, I believe.
You could also try with command line tool pdftops from http://aminet.net/package/text/misc/xpdftools_303_m68k which offers options to let you enter the password and saves to a non-protected PS file (which you can then convert back to a non-protected PDF file with Ghostscript).