Gmail has recently updated the icon for PDF attachments: it no longer uses the Adobe Reader icon and it opted for a basic text icon.
Here's the PDF icon used by Gmail since its release back in 2004:
Google Drive also shows the new icon for PDF files:
Now that browsers like Chrome and Firefox have built-in support for opening PDF files, Gmail and Google Drive let you open PDF files, Adobe Reader is less important and fewer people install Adobe's PDF software. That's probably the reason why Google picked a different icon.