DOC: File size reduction

docs/user/file-size.md

@@ -3,6 +3,33 @@
 There are multiple ways to reduce the size of a given PDF file. The easiest
 one is to remove content (e.g. images) or pages.
 
+## Removing duplication
+
+Some PDF documents contain the same object multiple times. For example, if an
+image appears three times in a PDF it could be embedded three times. Or it can
+be embedded once and referenced twice.
+
+This can be done by reading and writing the file:
+
+```python
+from PyPDF2 import PdfReader, PdfWriter
+
+reader = PdfReader("big-old-file.pdf")
+writer = PdfWriter()
+
+for page in reader.pages:
+    writer.add_page(page)
+
+writer.add_metadata(reader.metadata)
+
+with open("smaller-new-file.pdf", "wb") as fp:
+    writer.write(fp)
+```
+
+It depends on the PDF how well this works, but we have seen an 86% file
+reduction from 5.7 MB to 0.8 MB within a real PDF.
+
+
 ## Remove images