A client was having some issues where Adobe Acrobat Standard and Reader would not display some pages inside a PDF document. The PDF document was composed of a mix of pages cut and pasted from several PDF documents.
Turns out that the Acrobat PDF standard is a moving target and there are multiple authoring apps (even hardware document scanners) which may or may not comply to one of the standards - there is an Appligent document that explains the issues - and yes it is a PDF document.
Anyway, I found that Acrobat Reader and Standard X (10.1.1) refused to display the thumbnails and the pages in a Version 1.6 document but free NitroPDF Reader 2 (and other third party readers) would display those pages. One work around with some files was to use the free and Portableapped PDFtkBuilder to export the file to a Version 1.4 document. Whether this corrected the malformed PDF or Version 1.4 compliances are less rigid, I don't know.