OnCloudPDF

PDF glossary: 31 terms explained simply

This glossary defines 31 common PDF terms in one or two sentences each, grouped alphabetically. Where OnCloudPDF has a related tool, the entry links to it and states what the tool actually does and does not do.

Updated · Runs in your browser, no upload, no account

A to D

Annotation

An object added on top of a page's content, such as a comment, highlight, sticky note or form field. Annotations are stored separately from the page text. Flatten PDF is limited to form fields and does not specifically flatten comments or highlights.

Archival PDF (PDF/A)

A family of ISO standards for PDFs meant to be readable for decades, which restrict things like external fonts and encryption. OnCloudPDF's PDF/A tool is not certified: it turns each page into an image with archival-style metadata, and the conformance level you choose is only a label.

Bookmark

A clickable outline entry, usually shown in a side panel, that jumps to a page or section. Tools that rebuild a file from copied pages, such as Reorder Pages and Unlock PDF, may not carry bookmarks over.

Compression

Storing data in fewer bytes. Lossless compression keeps the content exactly; lossy compression, as used by JPEG images, discards detail. Compress PDF only optimizes the file's structure and does not recompress images, so it cannot trade quality for size.

Crop box

The rectangle of a page that viewers show. Content outside it can remain in the file. Crop PDF sets the crop box, so it hides margins but does not delete what lies outside it.

DPI (dots per inch)

A measure of image resolution: how many pixels are used per inch of printed size. A PDF page is measured in points (72 per inch), so converting it with PDF to Image at Low, Normal, High or Very High quality renders it at 1x, 2x, 3x or 4x, roughly 72, 144, 216 and 288 dpi for a standard page.

E to M

Embedded font

A font whose glyph data is stored inside the PDF so the document looks the same on any device. Without embedded fonts, a viewer substitutes another font and the layout can change.

Encryption

Scrambling the contents of a PDF so that it needs a password to open. It is different from permissions, which restrict actions on an openable file. Unlock PDF is not a password cracker, and truly encrypted files may come out unreadable.

Flatten

To turn interactive elements into fixed page content. Flatten PDF does this for fillable form fields, after which they cannot be edited; the text remains selectable.

Form field

An interactive element such as a text box, checkbox or dropdown that the reader can fill in. A form made of these fields is often called an AcroForm. OnCloudPDF can flatten form fields but has no tool to fill them in.

Grayscale

An image or page that uses shades of grey instead of colours. Grayscale PDF renders each page to a grey JPEG, so text becomes an image and file size can go up or down.

JPEG

A lossy image format common for photos. Scanned PDFs usually contain one JPEG per page. Images to PDF embeds JPEG and PNG files as they are, without recompressing them.

Metadata

Information about a document rather than its visible content: title, author, subject, keywords, creator, producer and dates. PDF Info reads it, and Edit Metadata can overwrite five fields but cannot clear them.

N to P

N-up

A layout that places N pages on one sheet. N-up PDF offers 2, 4, 6 or 9 pages per A4 landscape sheet and renders each page as an image.

Object

The basic building block of a PDF file's structure: pages, fonts, images and dictionaries are all objects with numbers, linked to each other.

Object stream

A way to pack many small objects together and compress them, which makes the file's bookkeeping smaller. This is what Compress PDF enables when it re-saves a file.

OCR (optical character recognition)

Software that recognizes text in an image and produces real characters. OCR PDF runs Tesseract in your browser in eight languages, one per run, and adds an invisible text layer to the page images; accuracy depends on scan quality.

Owner password and user password

A user password is needed to open a protected PDF; an owner password controls permissions such as printing or copying. Which tools respect those permissions is up to each program, so treat them as a deterrent rather than strong security.

Page box

A rectangle that defines part of a page, such as the media box (full page) or crop box (visible area). Page sizes shown by PDF Info come from the page size in points.

PDF/A

See Archival PDF above. It is a validated standard; a file is only PDF/A if it passes a validator for the claimed level.

Permissions

Flags inside an encrypted PDF that ask viewers to restrict actions such as printing, copying or editing. They rely on software cooperating with them. Unlock PDF copies pages to a new file without permission settings; use it only on files you may modify.

Point

The unit of PDF page measurement: 1/72 of an inch. An A4 page is about 595 by 842 points. Sign PDF positions your signature in points from the bottom-left corner.

R to Z

Rasterize

To convert text and vector content into a grid of pixels. Rasterized pages cannot be selected or searched, and a rasterized file can be larger or smaller than the original. Grayscale PDF, PDF to Scan, N-up PDF and PDF/A all rasterize pages.

Redaction

Permanently removing sensitive content from a document. Drawing a box, a stamp or a watermark over text is not redaction because the text stays underneath. OnCloudPDF has no redaction tool; see clean a PDF before sharing.

Resolution

The amount of detail in an image, measured in pixels. A higher resolution gives sharper pages and better OCR results on scans, but a larger file.

Signature: image versus digital

A signature image is a picture of a handwritten signature placed on a page, which is what Sign PDF adds. A digital signature is a cryptographic seal made with a certificate that can show who signed and whether the file changed afterwards; OnCloudPDF does not create one.

Stamp versus watermark

Both draw text on pages. A stamp is a status label such as Draft or Approved (Add Stamp has six presets); a watermark is custom text with your own size, opacity, rotation and colour (Add Watermark). Neither is a security feature.

Tagged PDF

A PDF with structure information (headings, lists, reading order) that assistive technology such as screen readers can use.

Text layer

The real, selectable text in a PDF, as opposed to text that is only a picture. PDF to Text and Word Count read this layer, so they return nothing for a scan until it has been through OCR.

Vector graphics

Shapes described by lines and curves, which stay sharp at any zoom, as opposed to pixel images. Rasterizing a page turns vectors into pixels.

XMP

Extensible Metadata Platform: an XML-based metadata block that can be embedded in a PDF alongside the standard document-information fields. Edit Metadata does not look for or remove it.

Frequently asked questions

What is the difference between OCR and a text layer?

A text layer is the real text stored in a PDF. OCR is the process that creates one from a picture of text. OCR PDF adds an invisible text layer on top of the page images so the file becomes searchable.

Is rasterizing the same as compressing?

No. Rasterizing converts text and vectors into pixels. It can reduce or increase file size and always makes text unselectable, while compression stores the same data in fewer bytes.

Is an image signature the same as an electronic signature?

Not necessarily. A signature image is a picture on the page. Whether it is accepted depends on the contract and the law that applies, and some contracts require a certificate-based signature that Sign PDF does not create.