IO 保存base64格式的PDF文件字符串后打不开文件提示不是正确的PDF文件

        plus.io.requestFileSystem(plus.io.PUBLIC_DOWNLOADS, function(fs) {  
            fs.root.getFile('test.pdf', {  
                create: true  
            }, function(fileEntry) {  
                console.log(fileEntry.fullPath)  
                fileEntry.createWriter(function(writer) {  
                    writer.onwrite = function(e) {  
                        plus.console.log("Write data success!");  
                    };  
                    var doc = new jsPDF();  
                    doc.text(20, 20, '你好 Hello world!');  
                    doc.text(50, 20, '你好 Hello world!');  
                    doc.text(20, 30, 'This is client-side Javascript, pumping out a PDF.');  
                    doc.addPage(); //生成下一页  
                    doc.text(20, 20, 'Do you like that?');  
//doc.save('Test.pdf');//如果用这行,下面的可以都注释起来,效果就是当前面页变为打开的PDF  
                    var abf = doc.output('dataurlstring'); //dataurlstring  
                    console.log(JSON.stringify(abf));  
                    console.log(abf.length);  
        abf = abf.replace(/^data:application\/(pdf|PDF);base64,/,"");//这里试过注释起来  
                    console.log(abf);  
                    //writer.seek(writer.length - 1);   //这里试过不-1 也试过注释起来  
                    writer.write(abf);  
                    //writer.abort(); //这里试过注释起来  
                })  
            }, function(e) {  
                alert("getFile failed: " + e.message);  
            });  
        }, function(e) {  
            alert("Request file system failed: " + e.message);  
        })  

用jspdf生成的base64格式的字符串保存为文件后打开生成的文件后提示不是正确的PDF文件
下面的是得到jspdf生成的base64格式的字符串
"data:application/pdf;base64,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"