Question

I want to see if the current tab is a PDF file from a background page.

I can check the url for .pdf at the end but there are some PDF files that don’t have that.

Answer 1

You can’t get it using current Chrome API afaik. What you can do is load this page again through XHR and check returned content-type header. Something like this:

background html:

chrome.tabs.onUpdated.addListener(function(tabId, changeInfo, tab) {
    if(changeInfo.status == "loading") {
        if(checkIfUrlHasPdfExtension(tab.url)) {
            //.pdf
            pdfDetected(tab);
        } else {
             var xhr = new XMLHttpRequest();
             xhr.open("GET", tab.url, true);
             xhr.onreadystatechange = function() {
               if (xhr.readyState == 4) {
                 var contentType = xhr.getResponseHeader("Content-Type");
                 if(checkIfContentTypeIsPdf(contentType)) {
                    pdfDetected(tab);
                 }
               }
             }
             xhr.send();
        }
    }
});

manifest.json:

"permissions": [
    "tabs", "http://*/*", "https://*/*"
]

For PDF files returned content type should be application/pdf. Something to keep in mind though is that content-type header could contain encoding as well: text/html; charset=UTF-8.