Earlier, it used to be a bit painful to view any PDF attachment in GMail. Google had a ‘View as HTML’ option for PDF’s, before users can decide to download the PDF.
The problem with this option was that the alignment would go wrong most of the times, and it was difficult for one to get the best view of what the PDF looks like before it can be downloaded. Also the other main problem was, earlier when the PDF’s were viewed as HTML, the images were stripped off the PDF’s to handle the loading of the file in an effective way. So if you were to get a PDF with lot of images, you’d pretty have no idea what the content was lest you downloaded it.
But now Google Mail has an in-built PDF reader, using which users can view the content of PDF without downloading it.
And the PDF looks like this when viewed internally.