This usually happens with scanned PDFS, due to a lack of embedded text in the document. Embedded text is what allows us to render your file as a collection of text rather than an image.
You can use a dot to annotate a scanned PDF, created by using the Comment Tool. You can create a dot by clicking somewhere on the document, instead of highlighting text.
To make the text readable, we have built an OCR tool that will process the document. You can find the OCR Tool here.