PDF Structure Inspector

Runs in browser PDF Analysis & Repair

For developers and support engineers: the trailer entries, a browsable catalog dictionary, objects counted by type, the page tree, stream counts and flagged findings — without a hex editor.

Privacy: Processed locally in your browser. Your file is not uploaded.

What is inside a PDF file, and what does repairing one do?

A PDF is a header naming its version, a body of numbered objects (pages, fonts, images, streams), a cross-reference table that records where each object starts, and a trailer pointing at the catalogue and the metadata. Metadata lives in the Info dictionary (title, author, dates, producer) and, since PDF 1.4, in an XMP stream; fonts are embedded whole, subsetted or only referenced by name. A viewer that cannot find an object walks the file to rebuild the cross-reference table — repair does the same and writes the rebuilt table, which fixes most "damaged file" errors without touching the pages.

Most blocks of the result map to a zone of the file: the trailer points to the catalog, and the catalog to the page tree. Conceptual model
Part 1:%PDF-1.7
Part 2:1 0 obj << /Type /Catalog /Pages 3 0 R >> endobj
Part 3:2 0 obj << /Title (Invoice) /Producer (…) >> endobj
Part 4:3 0 obj << /Type /Pages /Kids [4 0 R] /Count 1 >> endobj
4 0 obj << /Type /Page /Parent 3 0 R /Contents 5 0 R >> endobj
Part 5:5 0 obj << /Length 44 >> stream … endstream endobj
Part 6:xref
0 6
0000000000 65535 f
…
Part 7:trailer << /Size 6 /Root 1 0 R /Info 2 0 R >>
startxref
…
%%EOF
  • 2–5Body: numbered objects, counted by /Type in the object census
  1. 1HeaderNames the PDF version
  2. 2Catalog (/Root)The root dictionary; points to the page tree and almost everything elseCatalog tree
  3. 3Info dictionaryMetadata: title, author, dates, producer
  4. 4Page tree/Pages lists its /Kids; each /Page points to its contentPage tree statistics
  5. 5Content streamThe page’s drawing instructions, between stream and endstreamStreams
  6. 6Cross-reference tableWhere each object starts; repair rebuilds it
  7. 7Trailer/Size, /Root, /Info, plus /Encrypt, /ID or /Prev when the file has themTrailer rows

A minimal file with byte offsets left out. Many files keep a cross-reference stream instead of the table, or several trailers linked by /Prev; the inspector reads the last one.

How to use PDF Structure Inspector

  1. Drop a PDF.
  2. Click "Inspect".
  3. Expand the catalog tree and read the findings.

PDF Structure Inspector features

  • Header, trailer (/Size, /Root, /Info, /Encrypt, /ID, /Prev)
  • Catalog dictionary as a browsable tree (two levels)
  • Object census by /Type
  • Page tree statistics and stream count
  • Findings: JavaScript, OpenAction, launch actions, attachments, forms, multimedia, encryption
  • Read-only; local processing

PDF Structure Inspector example

Look inside a generated invoice

Input:

invoice.pdf

Output:

Header %PDF-1.7 · trailer /Size 48 · Root 1 0 R · Info 2 0 R · objects: Page 2, Font 3, XObject 1, … · findings: none

Frequently asked questions about PDF Structure Inspector

What is the catalog?

The root dictionary of a PDF (/Root in the trailer). It points to the page tree, outlines, names, metadata, viewer preferences and interactive features; almost everything hangs off it.

Why are some objects counted as "unknown"?

Objects without a /Type key — content streams, images referenced only by name, arrays — are grouped by what can be inferred; the rest are reported as untyped.

Can I edit objects here?

No, the inspector is read-only. Use Sanitize PDF to remove active content and the Metadata Editor for document properties.

Is my PDF uploaded to a server?

No. The file is opened and processed by your browser with WebAssembly and JavaScript; nothing is sent to Mutqan Soft or anyone else. Close the tab and the file is gone. Downloading the result needs a free account — no subscription, no payment.

Can I use it on a password-protected PDF?

Yes, if you know the password: the file list asks for it once and every operation then works on the decrypted pages. The output is saved without a password unless you protect it again.

Technical notes

The inspector reads the tolerant pdf-lib object table and the catalog, and takes /Size and /Prev from the last trailer or cross-reference stream in the bytes; nested dictionaries are described to depth 2 with names, numbers, strings, array lengths and stream markers. It never resolves /Parent or /Pages upwards, which keeps the tree finite.

References