Assuming this is not a follow up or a sequel of your other post, and assuming as well you are using either Photoshop or InDesign...
1. JPEG and PDF are/can be very different in nature, so the answers for one is not neccesarily the same for the other. This goes for a greyscale bitmap image (that means a JPEG but not always a PDF).
2. If the printer you refer to is not a human being ('The Printer') but a printing device, we do not know which one its is. Not that the precise model is needed but some kind of reference would help a lot... Because some printers have an inner setting that defaults (if chosen) to print greyscale images as a mixture of CMYK, supposedly 'neutral'. The one we use here for in-house printing (Minolta MagicColor 7450, a PostScript Laser, CMYK) is such, for example.
I guess that might be your case. If so, please, check your device software or LED control, as you could change this there.
If so, the trouble then is this: If you print only black, the results could be visually disappointing (though tone-neutral), because the print will lack the visual density and punch that is achieved by printing with more than one ink.
But, on the other hand, getting a real neutral greyscale printed with more than one ink is... difficult, to say the least. Even in comercial printing. But if you manage to do that, it is usually more pleasant that just one ink.
I don't know if that could be the case. Hope so.
For more ideas, we need a bit more information.
Gustavo, from Madrid
Thank you for your email. My non-human printer is a cannon CLC 5151 and the funny thing is that it does print in black sometimes.
I have a psd document with C=0 M=0 Y=0 K=various which I have created with the channel mixer presets. - I sent it to print and = 4 coloured grey result
I open an indesign document and place this very same file (plus others with the same presets with which I create my composition) - I send it to print and = black only
Then, I say great it's printing on black so now I just need to export this composition as a JPEG
So I export it and when I send to print = 4 coloured grey
The is when I realise that the C=0 M=0 Y=0 and various K pressets I originally created have been lost again on this JPEG.
I come back to indesign and export a PDF and I find that my original presets to black only are now divided into CMYK colour again.
Perhaps the channel mixer presets are only a guide and they are not preserved when I import the document anywhere else, but I wonder How come this psd document prints in 4 colured grey despite what you can see on the info?
I have tried to paste the image into the black channel of a new document but I can't save it as a JPGE nor I can place onto an indesign either because it doesn't recognize the colour mode.
I agree with you in that it's nicer to get a greyscale with 4 colour ink but only if the printer is properly calibrated and ours isn't at all. and I need to present both indesign and JPEG prints and they should be looking the same.
Saludos desde Londres
I am surprised you don't have a driver option for the printer to force it to use black ink only.
As for creating Black or grayscale only files use the Export as EPS Grayscale option in Indesign. You can them repurpose as a pdf or rasterize it on PS as a grayscale image. But this is still not going to help with a printer that you can't force to use black ink only.