How to combine two pdfs without losing any information?

My goal is to combine two PDFs. One has 10 pages, and another has 6 pages, so the output should be 16 pages. My approach is to load both PDFs into two NSData stored in an NSMutableArray.

Here is my saving method:

NSMutableData *toSave = [NSMutableData data];
for(NSData *pdf in PDFArray){
    [toSave appendData:pdf];
}
[toSave writeToFile:path atomically:YES];

However the output PDF only has the second part, which only contains 6 pages. So I don't know what did I miss. Can anyone give me some hints?

PDF is a file format which describes a single document. You cannot concatenate to PDF files to get the concatenated document.

But might achieve this with PDFKit:

1. Create both documents with initWithData:.
2. Insert all pages of the second document into the first one with insertPage:atIndex:.

This should look like:

PDFDocument *theDocument = [[PDFDocument alloc] initWithData:PDFArray[0]]
PDFDocument *theSecondDocument = [[PDFDocument alloc] initWithData:PDFArray[1]]
NSInteger theCount = theDocument.pageCount;
NSInteger theSecondCount = theSecondDocument.pageCount;

for(NSInteger i = 0; i < theSecondCount; ++i) {
    PDFPage *thePage = [theSecondDocument pageAtIndex:i];

    [theDocument insertPage:thePage atIndex:theCount + i];
}
[theDocument writeToURL:theTargetURL];

You have to add either #import <PDFKit/PDFKit.h> or @import PDFKit; to your source file, and you should add PDFKit.framework to the Linked Frameworks and Libraries of the build target in Xcode.