When I try to highlight a PDF, Zotero does not highlight the portion of text that I am moving the bar over. Instead, it will often highlight something from a previous page - usually a portion of some footnote or citation in the document. It's almost as if the PDFs I'm uploading are not able to be annotated except for those portions. I am using firefox browser and a Macbook. I am getting my PDFs from my school library - downloading the full text and uploading to my Zotero app. I am not sure what I may be doing wrong