// Marks are Redact annotations (pdfcraft-annot: Shape::Redact { quads, overlay }).
let marks: Vec<Mark> = marks(&doc);                 // page, areas (user space), fill, overlay text
let report = apply(&mut doc, None)?;                 // everything, or Some(&[pages])
clear_marks(&mut doc, None)?;                        // remove marks without applying

apply removes, under every marked area:

text: glyphs are cut out of Tj/TJ/'/" and replaced by a TJ displacement of the same advance, so the rest of the line keeps its exact position. Widths come from /Widths, /W//DW (composite fonts, embedded CMaps with codespace and CID ranges), Type 3 font matrices, and approximations for the standard 14. A glyph goes when it overlaps an area by more than a fifth of its size (or a point) both ways; zero-size text goes when its origin is inside;
images: fully covered → removed; partly covered → a copy with the covered pixels cleared (8/16-bit and 1/2/4-bit images, image masks; Flate/LZW/RL/A85/AHx). Codecs PdfCraft can't re-encode (DCT, JPX, JBIG2, CCITT) are removed whole (fail-closed); inline images under an area are removed. Images with separate /SMask or /Mask data, and images whose data decodes to more than twice their declared raster (plus a byte per row and 4 KiB), are removed whole; cleared copies drop /Alternates so alternate pixels cannot survive;
vectors: covered paths are removed; partly covered paths and shadings are clipped so nothing paints inside the areas;
form XObjects: rewritten recursively into new objects (pages sharing the original keep it);
annotations whose rectangle overlaps (with their pop-ups and their replies, /IRT), and form fields with a widget under a mark;
the marks themselves, replaced by boxes in their fill colour (and overlay text) drawn into the page as a /PCMark /Redaction stream.

A verification pass re-reads each redacted page and fails the operation if any glyph or inline image is still under an area. It checks every content stream the page had, whatever its dictionary calls it; only the overlay streams this apply added (Report::overlays) are skipped. Text is placed with the font and size set by Tf or by an ExtGState's /Font through gs, and a form XObject starts from the text state of the content that draws it. Content that can't be decoded makes the operation fail rather than leave it unredacted, and so does an inline image the renderer's tokenizer reads no image for (RedactError::Unreadable): where the page's operators resume after it would be a guess. A Redact annotation with neither a usable /QuadPoints array nor a usable /Rect makes it fail with RedactError::UnreadableMark before anything changes, instead of skipping that mark. The count covers the marks on the pages being applied.

Before changing any page, apply prepares the redaction appearances with a shared 8 MiB budget for their uncompressed PDF content across the selected pages. It bounds repeated-text counts and working line buffers before allocating them, and measures and encodes each repeated line once. Invalid or excessive appearances return RedactError::OverlayLimit without changing the document. Reduce the marked area, disable repeated text, or apply fewer pages at a time. This limits appearance generation; it is not a limit on the input PDF or the whole process's memory.

Structure cleanup resolves named MCIDs and indirect content references. Form content references are updated to the cleaned copy; unchanged sibling descriptions remain. The structure tree's object references (/OBJR) to the annotations and form fields apply removes are dropped, as Sanitize drops them, so a full rewrite of a tagged PDF can't keep a removed comment or field value. A tree where that can't be done (an element written in place, or one too large to walk) makes apply fail with RedactError::Tags rather than leave the annotation reachable.

Replacement descriptions (/Alt, /ActualText, /E) are removed from marked-content spans changed by cleanup, from every structure element above them (found through both /P and /K), and from elements that lose an annotation reference, which also lose /T. An element written in place above one whose descriptions go makes apply fail with RedactError::Tags. Unchanged spans retain their descriptions; unused property entries are retired during redaction and hidden-content cleanup, including aliases in shared/inherited resources. Descriptions still used by surviving pages or forms are kept.

A redacted page that inherits /Resources from the page tree gets them pinned (by reference; a direct inherited dictionary is made one shared object first) before it is rewritten. The parent's /Resources is then retired unless another page still inherits it, so it cannot keep a removed XObject reachable. Surviving form XObjects lose the resource entries for removed XObjects their own content doesn't draw, for the same reason. Pages that aren't redacted are left untouched.

Remove hidden information / Sanitize

let found = sanitize::scan(&doc);                          // [(Hidden, count)] per category
sanitize::remove_hidden(&mut doc, &[Hidden::Comments])?;   // chosen categories
sanitize::sanitize(&mut doc)?;                             // all of them

Categories: metadata (/Info, XMP), file attachments, comments, form fields (flattened so the values stay visible), hidden text (render modes 3 and 7, or wholly off the page), hidden layers (content of off optional-content groups, then the groups), bookmarks, links/actions/JavaScript, and /PieceInfo private data. XMP metadata, private data, associated files (/AF) and additional actions (/AA) are removed from every object reachable from the catalog that owns them (images, fonts, Forms, annotations and fields as well as the catalog and pages), never from name maps such as resource dictionaries. Content is read strictly: a page whose content can't be read in full is refused, not cleaned in part. Both make the next save a full rewrite.

patterns finds phone numbers, e-mail addresses, credit card numbers (Luhn-checked), US Social Security numbers and dates for Search & Redact.

codes holds the redaction code sets (U.S. FOIA and U.S. Privacy Act exemptions): CodeSet::from_id("foia")?.overlay(&["(b)(6)"]) gives the overlay text for the picked codes. Custom code sets aren't supported yet.

Not yet: pattern locales, struct-tree/alt-text cleanup, metadata and hidden-information sanitising (M8.4), re-encoding of DCT images (they are removed instead).