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 a Text Recognition tool that will process the document. You can find the Text Recognition Tool here.