PDF Terminology
A quick glossary of the PDF concepts Spark uses.
Page
A single rendered sheet. PDFs are a series of pages, each with its own dimensions and orientation. Spark identifies pages by 1-based page number in the UI (page 1 is the first page) but stores them 0-indexed internally.
Page Coordinates
Positions on a page are given in PDF user-space points, where 1 point = 1/72 inch. The origin (0, 0) is the bottom-left corner in the PDF spec, but Spark's display coordinate system puts (0, 0) at the top-left (the WPF convention). Spark's annotation data is stored in top-left-origin page-relative coordinates.
Tip — You rarely need to think about this. It only matters if you export annotation data to a third-party tool that expects bottom-left origin.
Annotation
A markup object attached to a page. Annotations include highlights, shapes, arrows, text boxes, sticky notes, stamps, signatures and redactions. They live in the PDF's annotation layer and are preserved across save / open.
Markup
Informal term for annotations you add as a reviewer — highlights, cloud shapes, callouts, sticky notes. Every markup is an annotation, but not every annotation is a reviewer markup (e.g. form fields, links).
Measurement
A special annotation type that stores real-world dimensions computed from a calibrated scale. Measurements include distance, perimeter, area, rectangle and circle measurements, and count markers.
Layer (Optional Content Group)
A PDF construct that groups content so it can be shown or hidden together. Architects use layers to separate disciplines (structural, mechanical, electrical). Spark's Layers panel lets you toggle layer visibility.
Form Field
An interactive element — text input, checkbox, radio button, dropdown, signature field. Form fields are editable directly on the page (the cursor changes to a text caret or pointer as appropriate).
Redaction
Marking content for permanent removal. Spark distinguishes Mark for Redaction (a reversible marker) from Apply Redactions (permanent burn-in). Applied redactions cannot be undone — save to a new file for safety.
Stamp
A pre-built or custom image / text marker (Approved, Draft, Reviewed, etc.) placed on a page. Stamps are first-class annotations — they move, resize and rotate.
Signature
Either hand-drawn with the mouse / pen (Draw Signature) or imported as an image (Import Signature). Applied as an annotation; can be tied to a cryptographic signature if the PDF requires one.
Snapping
When drawing measurements or shapes, Spark can snap to corners, midpoints and intersections of existing shapes or detected page features. Hold Shift while dragging to enable snap on the vertex editor.