Make sure you have the GhostScript plug-in enabled for working with PDF files.

1) Open the PDF file in "Advanced TIFF Editor".
File - Open - your PDF file

2) Make sure the page thumbnails panel is displayed.
View - Page and Thumbnails

3) Select the thumbnails of the pages to be deleted.
You can use the CTRL and SHIFT keys to group page selection.

4) Delete the selected pages.
Pages - Delete pages or just press the DELETE key.

5) Save the PDF file.
File - Save (if you want to save with the same name)
File - Save As (if you want to save under a different name)