It is worth noting that while PDF is popular, it is not the native format for digital comics. The industry standard is CBR (Comic Book Reader) or CBZ .
and subsequent volumes are available for purchase and immediate download to the Kindle app on Google Play Books Invincible Comics Pdf Download
While not every volume is included, Amazon frequently rotates "Invincible" compendia into its Kindle Unlimited subscription. You can read a third of the series for a $11.99 monthly fee, then cancel. It is worth noting that while PDF is