Guides
25 walkthroughs for the tools on this site — plus the background that explains why they behave the way they do.
PDFs & images
What Merging PDFs Keeps and What It Drops
Pages are copied rather than re-encoded — so quality survives and form fields do not.
How to Split a PDF Online for Free
Page-range syntax, and a one-step trick for reversing a backwards scan.
How to Delete, Reorder and Rotate PDF Pages
Why a page scans in sideways, and why rotating costs nothing in quality.
How to Convert JPG to PDF Online for Free
Photos into a document — page sizes, margins, and why your image looks small.
How Image Compression Works, and What It Strips
Why EXIF and GPS vanish, why PNG has no quality slider, and why transparency turns black.
HEIC vs WebP vs JPG vs PNG, Explained
Why iPhone photos won't open on Windows, and which format to convert to.
Data & developer
Why Your CSV Export Runs Formulas in Excel
CSV injection, why the header needs every key, and the byte Excel wants first.
How to Convert CSV to JSON Online
Semicolon delimiters, quoted fields, and the leading-zero trap.
How to Format and Validate JSON Online
The four things JSON does not allow, and how to read a parser error.
Base64 Encoding, Explained
What it is for, why it is not encryption, and the URL-safe variant in JWTs.
URL Encoding Explained: When and Why
Encoding a value versus a whole URL, and why a space is sometimes a plus.
How to Verify a File Checksum (MD5, SHA-256)
What a checksum proves, when MD5 is fine, and when it absolutely is not.
UUID v4 vs v7: Which Should You Use?
Why random IDs hurt database performance, and when v4 is still right.
Unix Timestamps, Explained
Seconds versus milliseconds at a glance, and why 2038 is a real deadline.
What Is Inside a JWT?
Signed, not encrypted — the three parts, the claims, and the alg:none attack.
Cron Expressions, and the OR Trap
Why 0 0 13 * 5 is not Friday the 13th, and what */5 really counts.
Regex, and Why It Sometimes Hangs
Catastrophic backtracking, greedy versus lazy, and the email-validation trap.
Text & generators
Why Word Counters Disagree on Your Text
Emoji count as two characters, Chinese as one word, and Dr. Smith as two sentences.
Every Text Case, and When to Use It
The real rules of title case, and which case belongs in your code.
How a Diff Actually Works
The longest common subsequence, and why moved text reads as delete plus add.
Making a Certificate That Actually Prints Right
Real A4 geometry, a 220 DPI capture, and why you print at 100% not fit-to-page.
How QR Codes Actually Encode Your Link
Static versus dynamic, why an uppercase URL makes a simpler grid, and print size.
What Actually Makes a Password Strong
Entropy rather than symbol soup, modulo bias, and what excluding look-alikes costs.