I like using libharu for my pdf needs. I have an older Qt4 project under a VM. It builds via vs2010 and everything works fine. If I clone the VM, remove Qt4, install Qt5 and try to build a project that uses libharu I get linking errors to zlib. Both projects use the same compiler and use the same libharu folder containing dll and includes. What could possibly be different?
Obviously what is changed is that now you have Qt5 instead of Qt4. The API is similar but not identical. I'm not very proficient in windows but from what you describe it looks like either you need to update the folder of the Qt library (configuration in visual studio) or you if libharu is looking for symbols exported by Qt4 and you don't have libharu source you will have to find an libharu implementation which uses Qt5. If you can build libharu yourself then you may try to patch it and adopt it for Qt5 but generally making such changes usually require some experience.