Faced the same issue, but resolved it myself.
- Right click on the file.
- select open with PDF view.
Now, the default is changed to PDF view from the Gallery earlier. When you click on any pdf file now, it'll open in a regular size and not in a small window.
If you do not have the Pdf viewer extension already installed, you may have to install / enable it.
However, after doing this, the image files opened in a small window. To resolve this, I once again changed the default to photos.
Now, both the Pdf as well as images open in a big window in google photos.
Hope this helps.