You'll need to ask in the apporpriate forum for whatever it is you are using. This one is for Adobe Reader. Reader can't extract pages.
OK. Thank you!!
Moved to Security & Electronic Signatures.
In short you cannot. One of the features of an electronic signature is that it assures the document has not been changed. The extracted page is not the orginal pdf. Hence no signature verification. If you want a single page with proof of signature, you can take a screen shot. It is really is not the signed page, but it is probably as good as you can get.
I see. But it's really inconvenient for me always need extract pages with electronic signature=.=
The thing you have to realize is when using Acrobat or Reader to create a digital signature you are signing the whole document (all of the bytes in the document). You are not signing just the page where the signature appearance resides.
Another thing to realize is the signature appearance (what you see on the page in the signature field) is not the signature itself. The signature proper is a block of hex encoded bytes written into the PDF file and not something you see (unless you open the file in a text editor). What you see on the page is just a graphical representation of the actual signature, but it's not the actual signature.
When you extract a page (even if it had the signature appearance on it) it is just a sub-set of the signed bytes, and if the signature were to be extracted as well it would be invalid because the bytes in the extracted file would not match the bytes in the signature. Because the signature in the extracted document would always be invalid we just remove it.