Javascript文件提交导致PHP脚本中的服务器错误

I've been going mad this past month trying to build a page to submit a pdf asynchronously to our server. I'm an experienced programmer, but never worked in web-dev or in php or javascript.

The javascript submission function is as follows:

function sendFile(event)
{
    $("#submitForm").hide();
    var formData = new FormData();
    var file=$("#journal")[0].files[0];
    formData.append("document", file);
    $.ajax("converter.php", { data:formData, processData:false, dataType:"text", method:"post", success:function(data, textStatus, jqXHR){
        if(data.length == 8)
        {
            $("#key_input").val(data);
            $("#key_submission").submit();
        }
        else
        {
            $("#response").html(data);
        }
    } });

It submits to the server but as soon the php script reaches this line

if(isset($_POST['document']))

the program crashes and I get an internal server error. What could be causing the crash?

This should be done with $.ajax() this way:

$.ajax({ //<------"{" an object should be passed here so you are missing a "{"
   url:"converter.php", //<-----"url:" was missing.
   data:formData, 
   contentType:false, //<-----this is also needed for file upload
   processData:false, 
   dataType:"text", 
   type:"post", //<------should be type not method 
   success:function(data, textStatus, jqXHR){
     if(data.length == 8){
            $("#key_input").val(data);
            $("#key_submission").submit();
     }else{
            $("#response").html(data);
     }
   } 
});

If you are trying to upload a file then contentType:false is needed with processData:false.

FormData
You can read more here.