You're right, there doesn't seem to be a ‘pure scan’ available, but there are endless copies of this: HiSoft C Manual for ZX Spectrum (pdf). While I don't share your aversion to OCR'd manuals (and having done a few myself, I'd not let one out the door unless it had fewer typos than the original …) you might wish to refer to the HiSoft C Manual for Amstrad CPC & CP/M: it is a scan, and seems very complete. You will have to refer to the OCR copy for Spectrum-specific routines, though.
I understand that HiSoft C was originally developed by compiling code using the CP/M BDS C system, then manually porting the result to each target Z80 system. The ZX Spectrum and Amstrad versions have very similar compilers, with the major differences being in the runtime library.