Why do some PDFs not have the preview option when uploaded?

[canvas_admin]

In the below screenshot, you can see a whole list of pdf files that have been uploaded and have the little pdf icon next to the file.  However, there is one file (HPCT_Anderson_RecommendedReading.pdf) that is a pdf but has a blank icon next to it. 

Whenever I use the sidebar to embed a file into a content page, I get the preview icon if the file has the pdf icon, but I DO NOT get the preview icon if the file has a blank icon next to it. See below.

I don't get it.  Why would some PDFs be previewable but one not be?   

[Stefanie]

 @canvas_admin , when I'm looking at your first screenshot of the Files space, I see that the icon to the left of the offending file is not a PDF icon. So, even though the extension is .pdf, could it be that the file itself is not--and that it is in fact a file type that the Box previewer cannot handle? Please check and let us know what you find.

[canvas_admin]

stefaniesanders​, it's definitely a PDF.  It was created in InDesign and exported as a PDF.   Adobe Pro opens it up as well as Adobe Reader. All of the Metadata also indicates it as a PDF.    From what I can tell, it's no different from the other PDFs in the list that DO have the PDF icon. :smileyconfused:

[Stefanie]

Hmm, thanks for the quick response,  @canvas_admin --I guess that would be too easy. The mystery continues, then, and I'll keep thinking about it, but I hope someone can come up with the answer.

[canvas_admin]

No worries.  I'm not immune to submitting a support ticket on this. I just thought I'd reach out here first to see if others had encountered this.

[RobDitto · Accepted solution]

By any chance, were these PDFs uploaded using a Firefox browser? We occasionally have a few Firefox users report the same plain white icon with PDFs they uploaded and can't preview (although I doubt our past cases involved InDesign).

Zachery Watson, a Canvas Support engineer, recommended this fix:

What to do if you can't download or save files | Firefox Help

Even though that solution is described as a fix to downloading, it also resolves an intermittent problem which a Firefox browser can develop: failing to send the "application/pdf" MIME type along with an upload.  (There's a different version of the same fix here.)

[canvas_admin]

 @RobDitto ​, that was it!  Brilliant!   Thank you.

[awilliams]

Nice! Community saves the day again.