Fix file corruption in your PDF
PDF file can become corrupt in a way that you do not notice as a lot of PDF viewers can fix or ignore such corruption. The SyncForce File Convert process detects such corruption and does not create thumbnails if it is present.
Luckily there is a fix that works most of the times (as long as the corruption is minor)!
Open the PDF file in Adobe Acrobat Reader and save the file again. Adobe Acrobat will fix minor corruption this way. After you've done this re-upload the file in SyncForce as a new revision to the same Digital Asset and when it does get through our corruption check, you've made sure you have stored an uncorrupted version of your PDF file.