PDF Glossary
Common PDF terms and definitions explained simply
A
AES-256
The Advanced Encryption Standard used with a 256-bit key. In a PDF, it scrambles the content. When the file has an open password, it cannot be read without that password; a file with only a permissions password opens without asking for one. Protect PDF uses AES-256 with an open password: the password is required to open the file.
Annotation
A note, highlight, comment or drawing stored on top of the page content rather than inside it, so a PDF reader can show, hide or delete it. Redact PDF removes the comments, links and form fields that a redacted area overlaps, with the replies to those comments, and leaves the others in the file, so check those before you share it.
B
Bookmark
An entry in the navigation panel of a PDF reader that jumps to a page or a section, also called an outline. Redact PDF does not change bookmarks, so a bookmark title can still show a name or a figure you redacted on the page.
C
Compression
Reducing the size of a PDF, mainly by resampling and re-encoding its images and removing redundant data. Compress a PDF offers three levels: on the server, images above 160, 140 or 120 dpi are brought down to 150, 120 or 96 dpi, except transparent or very small images and those that would shrink by 12% or less; real text stays sharp and selectable, and the result is never larger than the original.
D
Digital signature
A cryptographic signature based on a certificate, which lets software check who signed a document and whether it changed afterwards. Sign PDF does not create one: it places a visual signature, an image of your handwriting or typed name, with no certificate or timestamp.
DPI (dots per inch)
The number of pixels per inch of page, a measure of how sharp an image is. PDF to JPG and PDF to PNG render pages at 72 to 288 dpi (144 by default), and Compress a PDF can bring images down to 150, 120 or 96 dpi depending on the level.
E
Encryption
Scrambling the content of a file so that only someone with the key, here a password, can read it. Protect PDF encrypts a file with AES-256, and Unlock PDF removes the encryption when you enter the correct password.
F
Fillable form
A PDF with interactive fields, such as text boxes, checkboxes and drop-down lists, that you can complete in a PDF reader. Edit PDF can add X marks and a signature on top of a form, but it does not fill the fields themselves.
Flatten
Merging form fields and annotations into the page content so they can no longer be changed separately, often done before sharing a completed form. ProPDF has no dedicated flatten tool.
Font embedding
Storing the fonts a document uses inside the PDF, so it displays the same way on devices where the font is not installed. On the server, Compress a PDF tries to keep only the characters that are actually used, a technique called subsetting.
L
Linearized PDF
A PDF organized so that a viewer can show the first page before the whole file has downloaded, also called fast web view. It helps with large documents opened from a website.
Lossless compression
Compression that makes a file smaller without discarding any information, so the decoded image matches the original pixel for pixel. PNG is lossless, which suits pages with text and diagrams; JPEG discards some detail to produce smaller files.
M
Merge
Combining several PDF files into one document. Merge PDF works in your browser: you add the files, drag them into the order you want and download a single file named merged.pdf.
Metadata
Information stored in a PDF besides its pages, such as the title, author, dates and producing software. Redact PDF and Compress a PDF keep the title and author, and PDFs saved with pdf-lib name that library as their producer, so review the properties before you share a sensitive file.
O
OCR (optical character recognition)
Software that recognizes letters in an image, such as a scanned page, and turns them into real text. ProPDF has no OCR: Edit PDF, Chat with PDF and the Text mode of Compare PDF need a PDF with real text, while PDF to Word keeps scanned pages as images.
P
Page orientation
Whether a page is taller than it is wide (portrait) or wider than it is tall (landscape). Rotate PDF turns pages a quarter turn clockwise with each click, and JPG to PDF lets you choose portrait, landscape or automatic orientation.
Page range
A set of pages written as numbers and intervals, such as 1-3, 7, 12-15. In Split a PDF and Extract PDF Pages, a comma keeps pages in the same file and a semicolon starts a new file; pages are always saved in ascending order, without duplicates.
PDF/A
An ISO standard version of PDF designed for long-term archiving, which requires embedded fonts and forbids content that depends on outside resources. ProPDF has no tool to create or validate PDF/A files.
Permissions password
A second PDF password, also called the owner password, that controls actions such as printing, copying or editing, while the open password (user password) is needed to view the file. Protect PDF sets one password that serves as both, and printing stays allowed.
R
Rasterization
Converting vector content, such as text and shapes described by coordinates, into a grid of pixels. PDF to JPG and PDF to PNG rasterize each selected page, so the text in the resulting images can no longer be selected or searched.
Redaction
Removing confidential content from a document before you share it, rather than just covering it. Redact PDF removes the text under the areas you draw from the page, with the form fields, comments and links under those areas, so none of that text is left there to see, select or copy; the title, author, bookmarks and the files attached to the document stay in the file, and the accessibility tags of a tagged PDF lose the redacted text too, except in rare cases.
S
Split
Dividing one PDF into several files or taking some pages out of it. Split a PDF offers four modes: pick pages on thumbnails, type ranges, save every page as its own file or cut the file every N pages; several results are downloaded as a ZIP.
W
Watermark
Text or an image drawn across pages, such as DRAFT or a logo, to show a document's status or origin. It discourages reuse, but it can be removed with a PDF editor. Add a watermark works in your browser, on all pages or on a list such as 1-3, 5.
X
XFA (XML Forms Architecture)
An older Adobe form technology that describes a form in XML inside the PDF. The PDF 2.0 standard deprecates it, and ProPDF has no tool to fill XFA forms or any other form fields.