A page that looks the same everywhere
PDF stands for Portable Document Format. It was originally developed by Adobe in the 1990s and later became an international standard. The purpose was simple: a document should look the same whichever computer, phone or printer it reaches, regardless of what software or fonts that device has.
Most other document files do not promise that. A word-processor document is a set of instructions such as “this is a paragraph, use this font, wrap it to fit the page”. Open it on a device with different fonts or a different version of the software and the lines may wrap differently, the pages may break in other places, and the layout can shift.
A PDF takes the opposite approach. It records the finished page.
What is inside a PDF
A PDF describes each page as a set of drawing instructions: place these characters at these exact positions, draw this line, put this image here. Fonts used on the page are normally embedded in the file, so it does not depend on what is installed on your device. Images are stored inside it, usually compressed. The result is a fixed layout document, where position and appearance take priority over flexible structure.
Many PDFs also contain extras: links, bookmarks, form fields, comments, metadata such as the author and creation date, and sometimes attachments. Some are tagged with structure information that helps screen readers, although this depends on how the PDF was made.
Why editing is awkward
Because a PDF stores positioned pieces rather than flowing paragraphs, it generally has no notion of a paragraph at all. A line of text may be stored as several separate fragments. If you try to add a word, there is nothing that makes the following text reflow to make room, as it would in a word processor. Editing tools have to guess how the fragments fit together, and they sometimes guess wrong, which leads to odd spacing, broken lines or substituted fonts.
Changing a PDF is therefore much more fragile than changing the original document. When you can, edit the original file in the program that created it and export a new PDF. Many everyday tasks can be handled without true editing, though: filling in form fields, adding comments, highlighting, signing, merging pages, splitting out pages and rotating them are usually easier than changing the underlying text.
Why copying text can go wrong
Selecting and copying from a PDF can produce strange results, such as lines in the wrong order, missing spaces or unexpected line breaks. The reasons come from the same design. The file stores where characters appear, not how a reader would move through them, so software has to infer reading order, particularly in multi-column layouts and tables. Tables are especially troublesome, because a PDF has no real table structure unless it was specially tagged; it only has text placed in a grid-like arrangement.
Text PDFs versus scanned PDFs
Not all PDFs contain real text. If someone scanned a paper page and saved it as a PDF, the file may hold only a picture of the page. You can see it but cannot select or search the words, because to the computer it is just an image.
Optical character recognition, usually shortened to OCR, can analyze such pictures and add a hidden text layer. That makes the document searchable and copyable, but recognition is not perfect. Poor scans, unusual fonts and handwriting can all cause errors, so important details should be checked against the page itself.
Why PDFs can be large or slow
A PDF made from a high-resolution scan or a design full of large pictures can be big, because the images dominate the size. Reducing image quality or resolution usually shrinks it, at some cost to sharpness. Complex graphics and many embedded fonts can also make some PDFs slow to open or scroll on older devices.
Safety and privacy points
PDFs are generally safe to read, but like other files they can be misused. A fake invoice or a link inside a PDF can be part of online scams, so it is worth being careful with unexpected files; this is a common theme with email attachments. Some PDFs support embedded scripts or attachments, and well-maintained readers limit what these can do, which is one reason to keep your software updates current.
Redaction deserves a special warning. Drawing a black box over text in some tools only covers it visually; the text may still be underneath and could be selected or extracted. Use a tool with a proper redaction feature that removes the content, then check by trying to select or search for the hidden words. Likewise, metadata may reveal names, dates and software details you did not intend to share.
Password protection
A PDF can be set to require a password to open it, and some also carry restrictions on printing or editing. The open password is a real barrier when a strong password and modern encryption are used. Restrictions on printing or copying are generally weaker and should not be relied upon for anything truly sensitive.
When PDF is the right choice
PDF is well suited to things that should stay as they are: invoices, forms, contracts, manuals, tickets and anything to be printed. It is a poor choice for a document that others need to edit or that must reflow on small screens. Knowing that a PDF is a snapshot of a page, not a living document, explains most of its odd behaviour.