Duplicate Objects in Exported PDFs


I’ve got a few MS Word files (which I cannot, unfortunately, attach here due to not having rights to it) which I needed to convert into PDF format for usage. The original file is in .docx format, contains few images, and comes in at 2,227,762 bytes. I export to PDF using LibreOffice (x64) — not the latest version, to be sure, but reasonably close — using standard options, except for the addition of a document outline based on the title structure of the original document. Here are the options used:

The resulting document has 19,313,871 bytes (about 8.67 times as large as a PDF). Now, in some cases, one may wind up with a large PDF due to large images which are incorrectly (or not at all) compressed. But this strikes me as unlikely, given that such large images would make the original document large, as well. So I looked for another solution.

I downloaded a python application for PDF size optimization and applied it to the PDF. Below you will find the log:

C:\pdfsizeopt>pdfsizeopt.exe RR109.pdf RRv109-opt.pdf
info: This is pdfsizeopt ZIP rUNKNOWN size=69856.
info: prepending to PATH: C:\pdfsizeopt\pdfsizeopt_win32exec
info: loading PDF from: RR109.pdf
info: loaded PDF of 19313871 bytes
info: separated to 134065 objs + xref + trailer
info: parsed 134065 objs
info: eliminated 614 unused objs, depth=170
info: found 0 Type1 fonts loaded
info: found 0 Type1C fonts loaded
info: will optimize image XObject 44133; orig width=572 height=167 colorspace=/DeviceRGB bpc=8 inv=False filter=/FlateDecode dp=0 size=4946 gs_device=png16m
info: saving PNG to C:\Users\wtrmu\AppData\Local\Temp\psotmp.33668.img-44133.parse.png
info: written 4785 bytes to PNG
info: optimizing 1 images of 4946 bytes in total
info: executing image converter sam2p_np: sam2p -j:quiet -pdf:2 -c zip:1:9 -s Gray1:Indexed1:Gray2:Indexed2:Rgb1:Gray4:Indexed4:Rgb2:Gray8:Indexed8:Rgb4:Rgb8:stop -- C:\Users\wtrmu\AppData\Local\Temp\psotmp.33668.img-44133.parse.png C:\Users\wtrmu\AppData\Local\Temp\psotmp.33668.img-44133.sam2p-np.pdf
info: loading image from: C:\Users\wtrmu\AppData\Local\Temp\psotmp.33668.img-44133.sam2p-np.pdf
info: loaded PNG IDAT of 4505 bytes
info: executing image converter sam2p_pr: sam2p -j:quiet -c zip:15:9 -- C:\Users\wtrmu\AppData\Local\Temp\psotmp.33668.img-44133.parse.png C:\Users\wtrmu\AppData\Local\Temp\psotmp.33668.img-44133.sam2p-pr.png
info: loading image from: C:\Users\wtrmu\AppData\Local\Temp\psotmp.33668.img-44133.sam2p-pr.png
info: loaded PNG IDAT of 6353 bytes
info: executing image converter pngout: pngout -force C:\Users\wtrmu\AppData\Local\Temp\psotmp.33668.img-44133.sam2p-pr.png C:\Users\wtrmu\AppData\Local\Temp\psotmp.33668.img-44133.pngout.png
 In:    6410 bytes               C:\Users\wtrmu\AppData\Local\Temp\psotmp.33668.img-44133.sam2p-pr.png /c2 /f5
Out:    5908 bytes               C:\Users\wtrmu\AppData\Local\Temp\psotmp.33668.img-44133.pngout.png /c2 /f5
Chg:    -502 bytes ( 92% of original)
info: loading image from: C:\Users\wtrmu\AppData\Local\Temp\psotmp.33668.img-44133.pngout.png
info: loaded PNG IDAT of 5851 bytes
info: optimized image XObject 44133 file_name=C:\Users\wtrmu\AppData\Local\Temp\psotmp.33668.img-44133.sam2p-np.pdf size=4667 (94%) methods=sam2p_np:4667,parse:4940,#orig:4946,pngout:6063,sam2p_pr:6565
info: saved 279 bytes (6%) on optimizable images
info: optimized 624 streams, kept 41 #orig, 19 uncompressed, 564 zip
info: eliminated 52482 duplicate objs
info: compressed 1 streams, kept 0 of them uncompressed
info: saving PDF with 80969 objs to: RRv109-opt.pdf
info: generated object stream of 1169270 bytes in 80360 objects (7%)
info: generated 4684655 bytes (24%)

As you can see, the input PDF has 19,313,871 bytes distributed among 134065 objects + xref + trailer. Immediately, 614 unused objects are discarded. Then, a single image was compressed from 4,946 bytes to 4,667 bytes (saving 279 bytes). Then it eliminates 52,482 duplicate objects (39% of all objects in the file) and compresses 1 stream, writing the remaining 80,969 objects (1,169,270 bytes) to an optimised PDF file which has 4,684,655 bytes (24% of the original one).

Now, this isn’t properly a bug, since the PDF export routines do generate valid PDFs, but it is certainly a misfeature that those PDFs might have nearly 40% duplicate objects, and have such a structure that an optimiser can slash the size of the file to slightly under a quarter simply by discardinig duplicate objects (and possibly compressing some streams with DEFLATE).

So my question is, what is the procedure to request an improvement to the PDF generation so that the library which LO uses may try to consolidate objects to get its output to a sane size?

I will just cite:
Feature requests should go directly to Bugzilla as Enhancements

Bugs should be submitted directly to Bugzilla


Cited from the instructions for this site:

One additional hint: Bug reports work much better, if you provide an example file. I know you told your file can not be shared. Can you replace confidental text/data by something else? And do you know wich object is repeated so often in the document? (It may even be a side-effect of the conversion from a word-file…)

1 Like

Please take a look in Bug 157028 - FILESAVE PDF Tagged PDF export makes file size grow significantly


Thank you, this seems to be the bug I should be looking for.