Datacad's PDF's are vector generated PDF's in which that vector data can be pulled from them with the right tools, but I'd bet they are only looking for the searchable text aspect of it and have associated that with vector output comes the searchable text which Datacad does not give you from the output. You can use a program like PDF XChange Editor
to perform a OCR recognition on your PDF files which will add a searchable text layer over your existing text. It works very well, but the process does take some time to run.
I use the PDF Exchange PDF print driver to "print" our PDF's in a single multipage PDF which then can have OCR done on all the sheets in one batch from within the PDF XChange editor. If you have multiple PDF's to OCR then you would have to have something like the PDF Tool's app that came be bought as a standalone app
or comes with the PDF XChange Pro
pack that allows you batch OCR multiple PDF's at once.