I am using openSUSE Leap 42.2 By opening a Samba share and trying to open some PDF there Okular can not open them and shows error message for rejected access. If I download them locally Okular is opening them just fine. If I mount same share by smb4k then Okular can open the same documents without problems. Also if I open the share by krusader, Okular is opening same files without any problem. I think in past same problem has been reported and fixed, but unfortunately in this version of KDE appears again. I have reported this also to openSUSE.
Can you open the PDF file with any other programs, or is it just Okular that has issues? Also, is there any chance you can test with a distro or version that has newer packages? Leap 42.2 packages are quite old, and Leap 42.3 is now available.
Actually, just a dupe of https://bugs.kde.org/show_bug.cgi?id=361490 *** This bug has been marked as a duplicate of bug 361490 ***