The PDF file trailer (or cross-reference stream) contains a dictionary with a /Root entry — an indirect reference to the catalog object. For example:

<code>&lt;&lt; /Size 42 /Root 1 0 R /Info 2 0 R /ID [...] &gt;&gt;</code>

Object 1 is then the catalog:

<code>1 0 obj</code> <code>&lt;&lt; /Type /Catalog</code> <code> /Pages 3 0 R</code> <code> /Outlines 4 0 R</code> <code> /Names 5 0 R</code> <code> /AcroForm 6 0 R</code> <code> /ViewerPreferences &lt;&lt; /HideToolbar true &gt;&gt;</code> <code>&gt;&gt;</code> <code>endobj</code>

The catalog's /Type must be /Catalog. The most important entries:

<strong>/Pages</strong> (required) — An indirect reference to the root of the page tree. Every page in the document is reachable from this object. See <a href="/glossary/pdf-page-tree" target="_blank" rel="noopener">PDF Page Tree</a> for the page tree structure.

<strong>/Outlines</strong> (optional) — The root of the outline (bookmarks) tree. Contains a linked list of bookmark items, each with a /Title, /Dest or /A (action), and /First, /Last, /Next, /Prev pointers for the tree structure.

<strong>/Names</strong> (optional) — A dictionary of name trees providing named resource lookups: /Dests (named destinations), /JavaScript (document-level scripts), /EmbeddedFiles (file attachments), and others. See <a href="/glossary/pdf-name-tree" target="_blank" rel="noopener">PDF Name Tree</a> for the name tree structure.

<strong>/AcroForm</strong> (optional) — The document-level form dictionary for AcroForm interactive forms. Contains the /Fields array (all form fields in the document), /DA (default appearance), /DR (default resources), /XFA (XFA form data), and form flags. See <a href="/glossary/pdf-acroform" target="_blank" rel="noopener">PDF AcroForm</a> for interactive forms.

<strong>/ViewerPreferences</strong> (optional) — Controls default viewer behaviour: /HideToolbar, /HideMenubar, /HideWindowUI, /FitWindow, /CenterWindow, /DisplayDocTitle, /NonFullScreenPageMode, /Direction (reading direction), /PrintScaling, /Duplex, /PickTrayByPDFSize, /PrintPageRange.

The document catalog contains many more optional entries:

<strong>/MarkInfo</strong> — Indicates whether the document is a tagged PDF. Contains /Marked (boolean: true if the document has a complete tag structure), /UserProperties (boolean: true if user properties are present in the structure tree), /Suspects (boolean: true if some objects may not be correctly tagged). PDF/UA validation checks this entry. See <a href="/glossary/tagged-pdf" target="_blank" rel="noopener">Tagged PDF</a>.

<strong>/StructTreeRoot</strong> — The root of the document's logical structure tree (tag tree). Present in tagged PDFs. Required for PDF/UA. The structure tree maps the document's logical structure (headings, paragraphs, tables, figures) to the physical content streams.

<strong>/Lang</strong> — The natural language of the document as a BCP 47 language tag (e.g., "en-US", "de", "zh-TW"). Required for PDF/UA. Used by screen readers and text-to-speech engines for correct pronunciation and rendering.

<strong>/Metadata</strong> — An XMP metadata stream containing Dublin Core, PDF, and XMP Basic properties. The canonical source of document metadata (title, author, creation date, modification date, producer, creator tool). See <a href="/glossary/pdf-metadata" target="_blank" rel="noopener">PDF Metadata</a> for XMP vs Info dictionary details.

<strong>/OCProperties</strong> — Optional Content Properties dictionary. Present if the document uses layers (optional content groups). Contains the /OCGs array (all layer objects), /D (default configuration — which layers are on/off by default), and /Configs (alternate configurations).

<strong>/Perms</strong> — Document permissions (for encrypted documents). The /UR3 entry contains a usage rights signature enabling Reader Extensions (allowing Acrobat Reader to exercise features normally reserved for Acrobat). See <a href="/glossary/pdf-encryption" target="_blank" rel="noopener">PDF Encryption</a>.

<strong>/OpenAction</strong> — The action or destination executed when the document is opened. Can be a named destination (to open to a specific page), a GoTo action (explicit destination), or a JavaScript action (to run a script on open). PDF/A prohibits JavaScript OpenActions.

<strong>Incremental updates</strong> — When a PDF is saved with incremental update (the standard for signed PDFs and Acrobat "Save" operations), new versions of objects — including the catalog — are appended to the end of the file. The new trailer's /Root points to the new (updated) catalog object. The original catalog and original body remain intact before the update. This is the basis for PDF digital signature validation: the signed byte range includes the original body up to the signature field; the incremental update following the signature contains the signature value itself but must not change any signed content. See <a href="/glossary/pdf-digital-signature" target="_blank" rel="noopener">PDF Digital Signatures</a>.

<strong>Linearised PDFs</strong> — In a linearised ("optimised for web") PDF, the file is structured to allow page 1 to be rendered before the entire file is downloaded. Linearised PDFs have a special /Linearized dictionary as the first object in the file and a specific object ordering designed for byte-range delivery. The document catalog is written near the beginning of the file (within the first part), though it may be updated by a hint-containing append at the end. The /Linearized entry is removed if the document is subsequently modified without re-linearising.

<strong>Catalog validation</strong> — PDF validators check the catalog as part of conformance verification. PDF/A validators check /MarkInfo, /Lang, /Metadata, /StructTreeRoot, and the absence of prohibited entries. PDF/UA validators check /Lang and /MarkInfo specifically. PDF/X validators check the absence of /AcroForm interactive actions and JavaScript. The catalog is the starting point for all conformance checks because all document-level resources are reachable from it.