It seems the comic problem (see below) is related to a bug in Firefox version 49.0.1.
I work with Chrome which is OK but when I checked with Firefox 49.0.1 the comics do not display in the Firefox in-browser reader (other files seem OK?). It seems it’s a known fault with PDF downloads and Firefox 49.
Quote from Mozilla web site… “Troubleshoot problems with the PDF viewer. ….. With certain types of PDF files, the PDF Viewer may have problems displaying fonts, colors or the whole document. If some PDF files don’t render well or are blank, click the download button on the right side of the document header to open it with the default PDF viewer application on your computer.”
I’ll try to see if it’s something in the way I create PDF documents but in the mean time, all I can suggest is either download documents or use an alternative PDF viewer. Mozilla make some suggestions at….https://support.mozilla.org/en-US/kb/view-pdf-files-firefox#w_using-the-built-in-pdf-viewer