When PDF files are merged in Preview.app are there any drawbacks regarding compatibility with the PDF standard and Acrobat?

My procedure is to open two PDFs in Preview and drag the pages from the thumbnail drawer of one document to the other, then save. I also delete pages be selecting the page thumbnail and pressing 'delete'.

When PDFs are merged in this way, are they really merged into a single PDF or is the first PDF just acting as a container file for the second? Likewise, are deleted pages really removed from the PDF or just 'commented' out in the rendering engine?

  • I had used this technique regularly for Leopard through Mountain Lion.
    – Mike
    Commented Dec 9, 2014 at 21:47
  • I tried once more to be sure. On Mountain Lion this method: open PDF doc2, open PDF doc1, drag thumbnail from doc1 onto doc2. The doc2 thumbnails display pages from doc1 + doc2. But once saved this new doc2 does only contain doc2 pages.
    – dan
    Commented Dec 10, 2014 at 14:32

2 Answers 2


Merged PDFs from Preview are perfectly valid and follow the standard. I've never found any incompatibilities with them. It does merge it into a single PDF, and any pages that are deleted are removed from the file. You can test this by checking file sizes first before merging, then after merging, then after deleting some pages.

The only drawback I've found with Preview is that the Apple code for writing PDFs seems to default to writing PDFs which are faster for Preview to render, but which are larger than typically put out by Acrobat. (At the moment, I can't find the link that originally proposed this hypothesis... It's been a number of years.) For example, you might open an Adobe-created PDF in Preview, immediately save it to another file, and notice that it has expanded. This change in file size does not change the image quality in any way.

In general, the change in file size is not too much of a deal. It's only in rare cases (such as 120MB scanned PDFs) that I worry about which app I use to create the PDF.

  • I have noticed that the PDFs generated by my remote scanner (emailed to me) are smaller than when I re-save them with preview. I suppose I can always take my Apple PDFs to a machine with an Acrobat license at some point and convert them all to reduce the file size. I just wanted to be sure that I wouldn't regret using Preview at some point in the future.
    – Mike
    Commented Oct 18, 2013 at 23:49
  • Do you have a reference for "Merged PDFs from Preview are perfectly valid and follow the standard"?
    – Mike
    Commented Oct 18, 2013 at 23:50
  • 1
    I do not have a web reference but they pass Adobe Acrobats's preflight checks. Commented Oct 19, 2013 at 2:29

The PDF format is designed to be a container for page objects which can be added, deleted or reordered very easily.

By very crude analogy, you can easily add another Paragraph to an HTML document by adding a set of <p> tags around some new text data. You can remove the paragraph by deleting the tags and text. You can re-order the paragraph by moving its order relative to other paragraphs.

So, dragging pages from one PDF document into another simply adds the Page object data from the source document to the destination document. So there is no difference between 'really merged' and 'acting as a container for more page data'. Page data is indexed, so the PDF document knows how many pages it contains and in what order they come.

Similarly, deleting a page removes that Page Object from the PDF data stream and re-indexes the pages. You can test this by deleting a page and checking the file size reduction.

Obviously, Apple tries to conform to the PDF specification (otherwise they wouldn't be PDFs, they would be 'Apple files'.) Any divergence is a bug. However, the same page information can be written in a variety of ways: different encoding structures, colorspaces, additional metadata, etc, so it's possible for Adobe's PDFs and Apple's PDFs (and others) to be different sizes and 'styles' within the PDF spec.

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