What are PDF Xref tables?
Xref tables are part of the original PDF file specification and one of the features which gives the PDF file format its flexibility. It is found in the trailer. A PDF file can contain multiple linked xref tables which all need to be read. The pointer to the first xref table is usually found at the end of the PDF files in the last 1024 bytes. Linearized PDF files includes some data at the start of the file so you can display the file before it has been fully read.
If you open a PDF file in a text editor and search for the word ‘xref’ you will find something like this
xref 0 271 0000000000 65535 f 0000000015 00000 n 0000000102 00000 n
This is the xref table. A PDF consists of lots of COS objects and this tells you where they are located in the file. This is actually very useful. A PDF Reader just has to read these values and then it loads the objects only when they are needed. It does not need to parse or load the whole file.
The first line tells you about the table entries. In this case the xref table has 271 entries and the object numbers start at zero. The following lines give the object offset from the start of the file, then the generation number (you can have several revisions of an object) and a flag to say whether the object is in use (n) or not (f). If the PDF file has been edited and objects changed, the changed version is often tagged onto the PDF with an updated xref table showing the new location. So it is possible for a PDF file to contain several xref tables and the later values are used.
If you look at byte offset 15 in the PDF file I took the xref table from you will find the start of object 1
1 0 obj<</Type/Font ...
If you are looking at PDF file created with version 1.5 and above, you may not find an xref entry because they introduced an alternative way to store the objects inside Compressed streams.
Xref tables also explain why if you alter a byte or add a byte to a PDF file it will become corrupted – all the pointers are now wrong.
Is there an easy way to explore PDF Xref tables?
Our Java PDF Viewer has a inspection mode which you can use to view Xref tables and see what data they point to.
It works in trial version as well (so you do not need to buy our software to use it). Find out more…
Our software libraries allow you to
| Convert PDF files to HTML |
| Use PDF Forms in a web browser |
| Convert PDF Documents to an image |
| Work with PDF Documents in Java |
| Read and write HEIC and other Image formats in Java |
Does the xref list the objects in order?
It is in object order from the first number listed
As part of our process where I work, we split incoming PDFs into individual pages before archiving them. We came across a few that broke the library we used to split them. Looking closer at the xref table I noticed these weird offsets:
>grep -Eabo "[0-9]{10} [0-9]{5} (n|f)" "brokenpdf.pdf"
...
337232:0000225190 00000 n
337252:0032768500 00000 n
337272:0000225389 00000 n
337292:0000225565 00000 n
337312:0000225763 00000 n
337332:0000235424 00000 n
337352:0000235612 00000 n
337372:0000235815 00000 n
337392:0000245138 00000 n
337412:0000245318 00000 n
337432:0032768500 00000 n
337452:0000245520 00000 n
...
This offset in hex looks like 0x01f401f4, which is suspicious, plus the fact that the PDF itself was only 200K. Then I searched for all the objects in the document:
>grep -Eabo "[0-9]+ [0-9]+ obj" "brokenpdf.pdf"
...
225190:28 0 obj
225389:30 0 obj
225565:31 0 obj
225763:32 0 obj
235424:33 0 obj
235612:34 0 obj
235815:35 0 obj
245138:36 0 obj
245318:37 0 obj
245520:39 0 obj
...
Objects 29 and 38 were missing, and where their offsets would have been recorded in the xref table, there was a rediculously large number. Changing the “n” to “f” for these xref entries seemed to fix the issue well enough to split the document.
If I understand correctly on how to update an existing PDF to add a signature:
1. Get the root from trailer
2. Get the /Pages from root object
3. Get the /Kids from the pages object
4. Recreate the first kid reference, including an /Annots key to point to your objects.
Is that the proper procedure?
That would work in theory but you would need to write a new trailer to the end of the file with your new Annotation and with the new Root and Annots
Hi mark,
I’m looking at a file where “xref” starts with 1 instead of 0. It’s a file that has been checked and has no other “xref”. I would like to ask if I am facing a PDF that has been modified or forged?
Just to complement with more information, using exiftool, I got …
[…]
MIME Type : application/pdf
PDF Version : 1.7
Linearized : No
Warning : Root object (11 0 obj) not found at offset 3474428
— press ENTER —
Thanks in advance.
Or just broken. Does it open in Acrobat? What tool was it created with?
Yes, I can open the file without any problems. About the tool used in the creation of the file, all the pages of the file were generated by a scanner (without OCR), each one with a unique image.
Another strange point was the following: starting the object count with the number 1 (according to xref), the Root obj address is 0003474428, as shown in the message “Warning: Root object (11 0 obj) not found at offset 3474428 “. However, the obj found at that address is 10. Finally, /Info is out of information. With all this, is it possible to say that there was some manipulation of the file?
xref 1 13
3474846:0000000000 65535 f
3474866:0000000009 00000 n
3474886:0001414967 00000 n
3474906:0001415064 00000 n
3474926:0003474616 00000 n
3474946:0001415251 00000 n
3474966:0002852031 00000 n
3474986:0002852128 00000 n
3475006:0002852315 00000 n
3475026:0003474331 00000 n
3475046:0003474428 00000 n
3475066:0003474687 00000 n
3475086:0003474739 00000 n
9:1 0 obj
1414967:2 0 obj
1415064:3 0 obj
1415251:5 0 obj
2852031:6 0 obj
2852128:7 0 obj
2852315:8 0 obj
3474331:9 0 obj
3474428:10 0 obj
3474616:4 0 obj
3474687:11 0 obj
3474739:12 0 obj
12 0 obj
<>
endobj
It might have been tampered with or it might just have been created by a poor tool (see what the Creator/Producer says). Acrobat is very tolerant of errors in PDF files and will try to open anything. If you save from Acrobat, it should hopefully save a repaired version of the file.
So I can’t say that every original file (not altered, scanned or converted) should start xref with zero, right?
Depending on the tool, or the scanner used, there may be cases of original files (not updated) that the xref is started with other values?