how do i change the default program for opening pdfs in google chrome

There are at least two choices to read PDF documents with Google Chrome: opening PDF documents in the Google Chrome window itself or choosing which PDF reader program shall be started upon downloading a PDF document:

To read PDF (and PowerPoint) documents in the Google Chrome window, one can install an extension which displays these file formats using Google Docs. Adobe Reader or other plugins are not necessary. As another plus, one can open PowerPoint presentations (which some people still seem to make) without installing Microsoft Office or one of the likewise bloated pendants.

To choose what PDF reader is launched by Google Chrome upon downloading a PDF file, observe that Google Chrome (at least the Debian package provided by Google) uses the run-mailcap command to open downloaded documents. To change the default PDF reader to, say, xpdf, just create a .mailcap file in your home directory with the content

application/pdf; /usr/bin/xpdf '%s'; test=test "$DISPLAY" != ""; description=Portable Document Format; nametemplate=%s.pdf

application/x-pdf; /usr/bin/xpdf '%s'; test=test "$DISPLAY" != ""; description=Portable Document Format; nametemplate=%s.pdf

Now, Google Chrome should quickly launch xpdf on downloaded PDF files.