如何以编程方式添加pdf中的输入字段？ （iOS）

Question

如果无法在pdf中添加输入字段，我还有另外一个问题要问。是否有任何解决方案可以在已有输入字段的现有pdf中添加一些内容？

我可以使用以下代码合并pdf文件：

NSString *cacheDir = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];

// File paths
NSString *pdfPath1 = [[NSBundle mainBundle] pathForResource:@"pdf1" ofType:@"pdf"];
NSString *pdfPath2 = [cacheDir stringByAppendingPathComponent:@"temp.pdf"];
NSString *pdfPathOutput = [cacheDir stringByAppendingPathComponent:@"out.pdf"];

// File URLs - bridge casting for ARC
CFURLRef pdfURL1 = (__bridge_retained CFURLRef)[[NSURL alloc] initFileURLWithPath:(NSString *)pdfPath1];//(CFURLRef) NSURL
CFURLRef pdfURL2 = (__bridge_retained CFURLRef)[[NSURL alloc] initFileURLWithPath:(NSString *)pdfPath2];//(CFURLRef)
CFURLRef pdfURLOutput =(__bridge_retained CFURLRef) [[NSURL alloc] initFileURLWithPath:(NSString *)pdfPathOutput];//(CFURLRef)

// File references
CGPDFDocumentRef pdfRef1 = CGPDFDocumentCreateWithURL((CFURLRef) pdfURL1);
CGPDFDocumentRef pdfRef2 = CGPDFDocumentCreateWithURL((CFURLRef) pdfURL2);

// Number of pages
NSInteger numberOfPages1 = CGPDFDocumentGetNumberOfPages(pdfRef1);
NSInteger numberOfPages2 = CGPDFDocumentGetNumberOfPages(pdfRef2);

// Create the output context
CGContextRef writeContext = CGPDFContextCreateWithURL(pdfURLOutput, NULL, NULL);

// Loop variables
CGPDFPageRef page;
CGRect mediaBox;

// Read the first PDF and generate the output pages
NSLog(@"GENERATING PAGES FROM PDF 1 (%i)...", numberOfPages1);
for (int i=1; i<=numberOfPages1; i++) {
    page = CGPDFDocumentGetPage(pdfRef1, i);
    mediaBox = CGPDFPageGetBoxRect(page, kCGPDFMediaBox);
    CGContextBeginPage(writeContext, &mediaBox);
    CGContextDrawPDFPage(writeContext, page);
    CGContextEndPage(writeContext);
}

// Read the second PDF and generate the output pages
NSLog(@"GENERATING PAGES FROM PDF 2 (%i)...", numberOfPages2);
for (int i=1; i<=numberOfPages2; i++) {
    page = CGPDFDocumentGetPage(pdfRef2, i);
    mediaBox = CGPDFPageGetBoxRect(page, kCGPDFMediaBox);
    CGContextBeginPage(writeContext, &mediaBox);
    CGContextDrawPDFPage(writeContext, page);
    CGContextEndPage(writeContext);      
}
NSLog(@"DONE!");

// Finalize the output file
CGPDFContextClose(writeContext);

// Release from memory
CFRelease(pdfURL1);
CFRelease(pdfURL2);
CFRelease(pdfURLOutput);
CGPDFDocumentRelease(pdfRef1);
CGPDFDocumentRelease(pdfRef2);
CGContextRelease(writeContext);

但是当我尝试合并包含输入字段的pdf时，我得到了没有输入字段的普通pdf。那么在合并时如何将输入字段保存为pdf？感谢...