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# How pypdf parses PDF files
pypdf uses {class}`~pypdf.PdfReader` to parse PDF files.
The method {py:meth}`PdfReader.read <pypdf.PdfReader.read>` shows the basic
structure of parsing:
1. **Finding and reading the cross-reference tables / trailer**: The
cross-reference table (xref table) is a table of byte offsets that indicate
the locations of objects within the file. The trailer provides additional
information such as the root object (Catalog) and the Info object containing
metadata.
2. **Parsing the objects**: After locating the xref table and the trailer, pypdf
proceeds to parse the objects in the PDF. Objects in a PDF can be of various
types such as dictionaries, arrays, streams, and simple data types (e.g.,
integers, strings). pypdf parses these objects and stores them in
{py:meth}`PdfReader.resolved_objects <pypdf.PdfReader.resolved_objects>`,
populated by {py:meth}`cache_indirect_object <pypdf.PdfReader.cache_indirect_object>`.
3. **Decoding content streams**: The content of a PDF is typically stored in
content streams, which are sequences of PDF operators and operands. pypdf
decodes these content streams by applying filters (e.g., `FlateDecode`,
`LZWDecode`) specified in the stream's dictionary. This is only done when the
object is requested by {py:meth}`PdfReader.get_object <pypdf.PdfReader.get_object>`
which uses the `PdfReader._get_object_from_stream` method.
## References
[PDF 1.7 specification](https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/PDF32000_2008.pdf):
* 7.5 File Structure
* 7.5.4 Cross-Reference Table
* 7.8 Content Streams and Resources
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