Why can't I place annotations on a PDF
Our toolbar relies on the text information that is contained in the PDF to be able to place the annotations. Depending on how a PDF was created, it may or may not contain this text information.
PDFs generated from a digital document will almost always contain this text information. However PDFs which contain a scanned image will probably not contain it (for instance if you scanned a printed page into a PDF on your personal scanner). Some higher end scanners will attempt to add in the text information. Also, some publishers who have digitized their older content as scans of the journals, magazines, etc. will add this text information.
The simplest way to tell is to try to select the text in the PDF as if you were going to copy and paste it. If the PDF has the text information, you will be able to select the text. PDFs without this information won't let you make the selections.