>Does the cerebral spinal fluid of people with Alzheimer's have a notably different pH from 'normal' people's?

Probably not, but not for the reasons others have stated (which are true).

Excess Tau Protein in the CSF is the only reliable assay test we have for Alzheimers, because the Tau protein escapes into the CSF, but Alzheimers isn't a disease of the CSF. Alzheimers is a disease of Neuron Cells in the Brain.

The better question might be - is the Neuronal pH of Alzheimers disease patients different from nominally health people?

I also don't know the answer to this question either, but you might be able to find out. I wouldn't expect there to be a global difference in pH - homeostatis is very effective and long term pH changes are make for very unhappy cells (i.e. necrosis and death), but whether the pH changes during acute stress events enough to make a difference is an interesting thought that someone may have pursued.