To select an deleted all comments across pages ,please follow the instructions below:
1) Open the PDF file from which you want to delete annotations in Foxit PDF Editor (Pro)or Foxit PDF Reader.
2) Show Comments panel by going to View tab -- View Settings -- Navigation Panels>Comments.
or clicking "View document Comments" icon on the left side navigation toolbar.
3) Use one of the following optional methods to select comments that you would like to delete.
Method one:click anywhere in the Comments panel select one of the comments there first and then press "Ctrl + A" shortcut key on your keyboard to select all comments.
Method two: hold down Shift and click on the first and last comment to select all.
Method three: hold down Ctrl and click on the comments you want to delete one by one.
4) Right click on anywhere on the highlight area and click Delete to delete all of the selected comments.