Why put an apostrophe there as the plural of the word?
My guess is that CD and PC are initialisms, and some people seem to think they need an apostrophe before the plural s. They don't.
There is, unfortunately, varying advice about this, especially on the web. I take the orthodox view that follows, which is from Oxford Dictionaries:
The general rule is that you should not use an apostrophe to form the plurals of nouns, abbreviations, or dates made up of numbers: just add -s (or -es, if the noun in question forms its plural with -es). For example: euros, pizzas, the 1990s.
There are one or two cases in which it is acceptable to use an apostrophe to form a plural, purely for the sake of clarity:
You can use an apostrophe to show the plurals of single letters: I've dotted the i's and crossed the t's; find all the p's in appear.
You can use an apostrophe to show the plurals of single numbers: find all the number 7’s. (Yes, this contradicts what I wrote above, where I said you shouldn't, You can
do this, but you don't have
These are the only cases in which it is generally considered acceptable to use an apostrophe to form plurals. An apostrophe should never be used to form the plural of ordinary nouns, names, abbreviations, or numerical dates.