Please see the screenshot 2 beneath, after my failed attempt to fulfill Step 3:
- Search for Convert All Spots to CMYK and run Fix. Now all colors in your document are CMYK.
As advised by the answerer, I had to create this fixup, as my Adobe Acrobat DC Pro lacked it.
Your PDF doesn't have embedded color space (no ICC profile attached, no OutputIntent defined) so this error will come out for undefined colorspace objects. But for your particular case it doesn't matter, as you want to convert Pantone to default CMYK (probably US SWOP).
