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PDF to Text, a challenging problem

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357 points ingve | 1 comments | 13 May 25 15:01 UTC | HN request time: 0.203s | source
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1vuio0pswjnm7 ◴[13 May 25 18:28 UTC] No.43976096[source]▶
Below is a PDF. It is a .txt file. I can save it with a .pdf extension and open it in a PDF viewer. I can make changes in a text editor. For example, by editing this text file, I can change the text displayed on the screen when the PDF is opened, the font, font size, line spacing, the maximum characters per line, number of lines per page, the paper width and height, as well as portrait versus landscape mode.

   %PDF-1.4
   1 0 obj
   <<
   /CreationDate (D:2025)
   /Producer 
   >>
   endobj
   2 0 obj
   <<
   /Type /Catalog
   /Pages 3 0 R
   >>
   endobj
   4 0 obj
   <<
   /Type /Font
   /Subtype /Type1
   /Name /F1
   /BaseFont /Times-Roman
   >>
   endobj
   5 0 obj
   <<
     /Font << /F1 4 0 R >>
     /ProcSet [ /PDF /Text ]
   >>
   endobj
   6 0 obj
   <<
   /Type /Page
   /Parent 3 0 R
   /Resources 5 0 R
   /Contents 7 0 R
   >>
   endobj
   7 0 obj
   <<
   /Length 8 0 R
   >>
   stream
   BT
   /F1 50 Tf
   1 0 0 1 50 752 Tm
   54 TL
   (PDF is)' 
   ((a) a text format)'
   ((b) a graphics format)'
   ((c) (a) and (b).)'
   ()'
   ET
   endstream
   endobj
   8 0 obj
   53
   endobj
   3 0 obj
   <<
   /Type /Pages
   /Count 1
   /MediaBox [ 0 0 612 792 ]
   /Kids [ 6 0 R ]
   >>
   endobj
   xref
   0 9
   0000000000 65535 f
0000000009 00000 n 0000000113 00000 n 0000000514 00000 n 0000000162 00000 n 0000000240 00000 n 0000000311 00000 n 0000000391 00000 n 0000000496 00000 n trailer << /Size 9 /Root 2 0 R /Info 1 0 R >> startxref 599 %%EOF
replies(3): >>43976133 #>>43976276 #>>43980858 #
1. jimjimjim ◴[14 May 25 04:36 UTC] No.43980858[source]▶
This is the "Hello World" of PDFs.

Most pdfs these days have all of the objs compressed with deflate.

and then, because that didn't make it difficult enough to follow, a lot of pdfs have most of the objects grouped up inside object stream type objects which then get compressed. So you can't have text editor search for a "6 0 Obj" when you are tracking down the end of a "6 0 R"