You've not told us how you're viewing the pdf document. (Originally everybody used Adobe Reader but many people now prefer to use other programs, such as Foxit or SumatraPDF. Modern web browsers, such as Firefox, also let you view pdf files within a browser window). Sometimes you need to click a button, possibly at the foot your screen, to navigate from one page to the next.
However, try saving the pdf file to your hard drive (without viewing it) first: Go to the email that had the file as an attachment and select the option to save it, rather than to view it.
Then double-click on the file saved, so that it opens in your default pdf viewer (rather than within a browser window). If you still can't see the missing pages, try looking for a button to move between them (and/or simply printing out the document anyway, to see if the missing sheets actually get printed).