PDF Copying. Help in saving or coping one page within a multi page document

I need some help with adobe PDF.  I work in accounting and after I process a check run; I  like to save the invoice and copy of the check under the vendor file on the computer. in an effort to save time; I usually scan in all of the invoices at one time. then I would open the large PDF document and individually save each page or invoice under the vendor file  on my computer;  but for some reason I cant do this anymore.   Once I open the document and select the first page; I right click and select "save image as" - I go to the folder on the computer I am looking for and hit save but Adobe is only giving me the option to save it as a >bmp  >jpg or >tiff.   it doesnt allow me to save it as a PDF.  I have tried saving it as all three but when I go back to open it - I cant see the document because adobe is saving it as a picture.

I do not have time to scan in documents individually - PLEASE HELP -  explain how to copy and save one page out of a multipage document. thank you