PDF knowledge base
Understand the document before changing it.
Practical explanations for choosing a PDF workflow, checking a result and avoiding common mistakes.
What is a PDF?
A PDF describes pages so that text, graphics and images can be displayed independently of the application that created them. A PDF page may contain selectable text, a scanned picture, or both.
Read guide →PDF compression: what actually gets smaller?
A PDF can be large because of photos, embedded fonts, duplicated resources or object storage. Each cause needs a different optimization.
Read guide →Why PDF file size varies
Two PDFs with the same page count can have very different sizes. Page count alone is a poor estimate of storage or browser memory requirements.
Read guide →PDF versus Word: sharing and editing
Choose Word when a document needs ongoing editing and PDF when you want to distribute a finished page representation. Neither format automatically guarantees privacy or correctness.
Read guide →Choosing PDF or DOCX for a job application
A job application is a practical example of how file format affects a reader. Follow the employer’s instructions before choosing either format.
Read guide →OCR: turning a scan into text
Optical character recognition estimates characters from image pixels. It is useful when a scanned page has no selectable text, but its output requires proofreading.
Read guide →PDF privacy, encryption and redaction
Password protection, metadata removal and redaction solve different problems. Select a tool based on the information you need to protect.
Read guide →What PDF password encryption does
PDF encryption protects document content with a password-based cryptographic mechanism. An open password is different from a label saying CONFIDENTIAL.
Read guide →Electronic signatures and digital signatures
A typed name on a PDF is a visible electronic mark. A certificate-based digital signature is a cryptographic mechanism. They should not be described as the same feature.
Read guide →Filling and flattening PDF forms
An interactive PDF form stores field definitions and values. A scanned form may only be a picture of empty boxes and needs a different editing workflow.
Read guide →PDF fonts and missing characters
A PDF can reference a font or embed font data. Embedding helps reproduce glyphs when the recipient does not have the original font installed.
Read guide →PDF page sizes and points
PDF tools commonly use points for page geometry. One point is one seventy-second of an inch. A4 is approximately 595 by 842 points; US Letter is 612 by 792 points.
Read guide →PDF accessibility: more than readable pages
A PDF that looks clear can still be difficult to use with assistive technology. Reading order, tags, language, headings and alternative text can all matter.
Read guide →PDF/A: archival conformance needs validation
PDF/A is a family of PDF standards for long-term preservation. A filename ending in -pdfa.pdf or a producer label saying PDF/A is not evidence of conformance.
Read guide →PDF color: why screen and print can differ
PDF content can use different color spaces and profiles. The same numeric color can look different on a display and a printer because the devices reproduce color differently.
Read guide →PDF versions and feature compatibility
A PDF version identifies the format revision associated with the document. It does not, by itself, guarantee that every viewer supports every object or optional feature in the file.
Read guide →Linearized PDF and fast web viewing
Linearization organizes a PDF so a compatible reader can begin showing useful page content before downloading the whole file. It is also called fast web view in some applications.
Read guide →PDF layers and optional content
Some PDFs contain optional content groups, often called layers. A reader can show or hide these groups without necessarily deleting the underlying data.
Read guide →Inspecting PDF metadata before sharing
Document properties can reveal a title, author, subject, keywords, software name or dates. Additional information can also exist in XMP streams and other objects.
Read guide →Why PDF workflows still depend on source files
PDF became widely used because it lets people distribute a page representation across different computers and applications. Its strengths for sharing also explain some editing limitations.
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