PDF basics

Short lessons that explain how PDF files work. Read them in order, or jump to what you need.

1

What Is a PDF File?

What PDF stands for, why it was invented, and why a PDF looks the same on every device.

2

PDF Versions and PDF/A Explained

PDF 1.4, 1.7, 2.0, PDF/A, PDF/X, PDF/UA — what the versions and sub-standards mean and when they matter.

3

Scanned PDF vs Text PDF: What's the Difference?

Why some PDFs let you select and search text while others don't — and how to tell which kind you have.

4

Fonts in PDF Files: Embedding and Subsetting

How PDFs store fonts, what 'embedded' and 'subset' mean, and why missing fonts cause garbled or substituted text.

5

Vector vs Raster: How Images Work in PDFs

Why some PDF graphics stay sharp at any zoom while others get blurry, and how image resolution drives file size.

6

What Is OCR and How Does It Work With PDFs?

OCR (optical character recognition) turns pictures of text into real, searchable text. Here's how it works and where to get it free.

7

How PDF Compression Works

What actually happens when you 'compress' a PDF, the difference between lossless and lossy compression, and how much smaller files can get.

8

PDF Passwords, Permissions and Encryption

The two kinds of PDF passwords, what permissions like 'no printing' really do, and how strong PDF encryption is.

9

PDF Forms: Fillable Fields, AcroForms and XFA

How fillable PDF forms work, why some forms can't be filled in or saved, and the difference between AcroForm and XFA forms.

10

Electronic vs Digital Signatures in PDFs

The difference between drawing your signature on a PDF and a cryptographic digital signature, and which one you need.