1) This was true long before pdfs were invented, to the point that in the 90s, they more or less openly admitted that their published adventures were written to be read, not played. (And they don't admit it now, but I rather strongly feel like it's still true, otherwise a lot of the glaring problems with a number of their adventures wouldn't exist.)
2) WotC is not selling pdfs currently, but they do make all of their books available digitally at a significant discount, so even if you were entirely correct about it not being very much of a thing with physical books, it would still apply perfectly well to modern D&D anyway.