Camscanner is one of the most popular scanner apps sold round the world. For those with many pages to scan, or need higher quality images, the app is often used with a scanning stand like ScanJig Pro. It would also help to have a default page size most often used in your country.
One adjustment is needed for those who need to operate with letter sized (8.5 x 11) paper size. The app defaults to A4 paper size for PDFs. As a result, each time you scan a job the paper size needs to be changed to Letter.
To work around this you can change the A4 size specification to 8.5 x 11. Go to Settings - Scan Settings - PDF Size - select A4 and inches. Change the size to 8.5 x 11.
Now each PDF will be created in the 8.5 x 11 size.
Hopefully the folks at INTSIG, the makers of Camscanner, will include the ability to change the default in a future release.