Under the Image menu, choose Mode and ensure that the color is set to CMYK, as shown below.
Save File As PDF
From the File menu, choose Save As...
Select Photoshop PDF in the Format option, as shown below.
Name your PDF file and click Save.
Once you have named your PDF file, you will then be prompted to select the PDF settings.
Select [PDF/X-1a:2001] from the Adobe PDF Preset option, as shown below.
Simply click on the Save PDF button.