I have 3 files about 40 KB each. When I combine them using Preview and save them as a single PDF file, instead of having a size close to 120 KB, the size is closer to 500 KB.

Why does this happen, and is there any way to retain a small file size when the files are combined?

  • Does Merging files online reduce in size gradually . When I did it I found my first document a normal one but the 3rd & 4th was so tiny . Please give me a source , it's highly important.
    – Answer pls
    Aug 2, 2017 at 21:13

5 Answers 5


PDFs are funny creatures because Postscript is a very "fat" language; the same PDF can be written multiple ways. I would say that the extra code is probably glue code to put those three documents together and extra bits placed there by whatever utility was used to combine the three.

The best way to get rid of this would be to open the PDF in Acrobat Professional (not Reader) and run it through the "Reduce File Size..." command. This strips out any dead or unnecessary code, optimizes fonts (if two instances of the same font exist, the PDF is rewritten so that only one instance exists), and the like.

Ghostscript probably has a function that does something similar, but off-hand I'm not sure which one that would be.

  • 3
    In preview, "Save as..." has a "Reduce File Size" option under the "Quartz Filter" dropdown, in case you don't have and don't want to spend hundreds on Acrobat Professional. I don't know how the quality/compression this options compares, however.
    – kmm
    Feb 17, 2011 at 15:29
  • @Kevin: I didn't know that but thanks for the tip. I'm in book publishing, so Acrobat is always within easy reach. Feb 17, 2011 at 16:08
  • @Kevin: Thanks for the tip. I tried it, but it's quality is much lower than the original. Acrobat Standard's reduce file size maintains the some quality, but doesn't reduce the file by much. It seems that the trick is to combine the file in Acrobat rather than Preview (see my answer).
    – Senseful
    Feb 17, 2011 at 19:14
  • Another option is to use the free pdftk (pdf toolkit), which is a command line tool. It is an amazing tool for dealing with pdf files
    – hdrz
    Aug 3, 2017 at 18:30

Preview's "Reduce File Size" feature takes a 512KB file down to 254KB, but makes the text very blurry and difficult to read.

On Acrobat Standard (Windows), the "Reduce File size..." feature takes the 512KB file down to 418KB, but maintains the exact same quality that the original had.

Therefore, reducing the file size after the file was combined is not a good option. A better option is to use a more efficient program to combine the files...

I found out that Acrobat Standard is better than Preview at combining PDF files and maintaining a small file size. For example, if you have two 61KB files to combine, Preview will produce a 311KB file, while Acrobat (with the "Default File Size" option enabled) will produce a 106KB file.


This is similar to Nate Stevens' answer. For anyone who still has this problem, I had used the "insert" option from the edit menu, and got a file size about 6 times larger than the sum of the individual file sizes.

However, when I simply dragged and dropped the pdfs into the sidebar (thumbnail view), I got just a little larger than the sum of the individual file sizes.


I suspect this is caused by images rendered by Preview and re-encoded in a lower compression level. How was your PDF created in the first place? Another possibility is that characters where seen as character in the original file, but are rendered as bitmaps in the produced PDF.

  • The possiblility of the second item is very less...
    – Nivas
    Feb 17, 2011 at 13:09

I know that I am years too late to help the author of the original post but I just had this problem and figured it out. I had combined multiple PDF files in preview and they were appearing all different sizes. To fix the sizing go to print the document and instead of printing save as PDF through the print menu. This will give you a new pdf with all pages sized correctly but you will no longer be able to edit any of the text so make all the changes you need before you consolidate the sizing.

Good luck to anyone having this problem. I hope that I have helped.

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