If you only have access to the PDF and can't really open it in Illustrator/Photoshop to convert the spot colors (since most designers don't vectorize their fonts...), you can convert everything to CMYK in Adobe Acrobat Pro with the Preflight.
You can use the "Preflight" feature and create a profile to "fix" files; it will verify if you have any other color mode in your PDF but will also offer you to fix this to CMYK. So far, I always had excellent results with this.
There's already profiles you can choose from and that you can modify as well if you don't want to add any color profile to your conversion or remove all overprint black, increase line thickness, etc. There's lot of options. I find it very practical to have my own custom profiles with basic touch-ups only; I have one for grayscale and one for CMYK, but you can create as many as you want and it's a very quick way to solve your issue.
Of course, Adobe Acrobat Pro will use the default values of the Pantones, but that's the price to pay for the designer who sent you these files and didn't convert them; and frankly, there's so much variation of colors on digital press, there's no point in going crazy with the conversion as long it looks very similar to the original Pantones... which is usually the case.
You can always verify the result using the "output preview" tool that shows you the color values on your file. That's something you might want to verify if you have to convert an ad for Pfizer, for example!
PS: Converting the pantones yourself with this is still better than sending them to the digital printer and let it convert them. From experience, I can tell you the colors look way closer to the real color if you do it yourself this way; A lot of digital printers need to have their "pantones equivalent" calibrated and not many do it because it's a demanding job and not really worth it. Their Pantones equivalent is also most of the time very "off".