Ive been running into this problem ever since I updated the module to 7.x-1.1 and also updated the pdf.js to the current version in git.
Fortunately, Ive managed to fix it for myself changing the line 62 from:
$file_url = file_create_url($variables['file']->uri);
$file_url = file_create_url($variables['file']['uri']);
I hope this can help anyone with a similar problem.