31 guides
How PDFs actually work
Most PDF frustrations come from one of a handful of things nobody explains: that a scan and a document look identical but behave nothing alike, that a PDF stores positions rather than paragraphs, that one of the two password types is not really a lock. These are the explanations, written to be read once and then not needed again.
Each one ends at the PDFLove tool that does the job — all 36 of them free, and all of them running on your own device rather than a server.
Problems, explained
Things that go wrong in practice, and what is actually causing them.
- 01Why Your PDF File Is So LargeText costs almost nothing to store. Images cost almost everything. If your PDF is large, it is carrying pictures — even if it looks like a document.
- 02How to Email a PDF That's Too BigThe limit you were told is not the limit you get. Email encoding inflates every attachment by about a third before it is measured, so a 25 MB cap is really about 18 MB of file.
- 03Scanned PDF or Text PDF? How to TellTry to select a word. If the cursor picks out the word, you have text. If it draws a box over the whole page, you have a photograph — and that single fact explains most of what a PDF will and will not let you do.
- 04Why PDF to Word Loses FormattingA PDF records positions, not paragraphs. Converting to Word means reconstructing a structure that was thrown away when the PDF was made, which is why the result is a good approximation rather than the original.
- 05Does Merging PDFs Reduce Quality?Merging is a copy, not a conversion. The pages in the combined file are the same objects that were in the originals, so there is no quality to lose — a merged PDF is byte-for-byte as sharp as its sources.
- 06The Two Kinds of PDF PasswordPDF supports two different passwords doing two different jobs. One is real encryption. The other is a polite request that most software honours and nothing enforces.
- 07Flattening a PDF vs Printing to PDFBoth are used to lock a document down. Flattening merges the interactive layer into the page and leaves everything else alone. Printing to PDF rebuilds the whole file, and what survives depends entirely on the driver.
- 08A4, Letter, and Legal Page SizesA4 and Letter are close enough to look interchangeable and different enough to break a layout. A4 is 6 mm narrower and 18 mm taller, which is why documents from abroad print with clipped edges or an unexpected blank strip.
- 09Are Electronic Signatures Legally Valid?In the US, the EU, the UK and most comparable jurisdictions, an electronic signature has been legally valid for over twenty years. The interesting question is not whether it counts but what you would need to prove if it were challenged.
- 10What OCR Gets Wrong, and How to Fix ItOCR is recognition, not conversion, so it has an accuracy rather than a result. Most of what determines that accuracy was decided before the file reached any software — at the scanner.
- 11What Happens When You Upload a PDFMost online PDF tools work by receiving your document. That is not sinister — it is how the architecture works — but it does mean the file exists somewhere you do not control, for a period you do not set.
- 12JPG, PNG, or WEBP: Which Should You Use?The choice comes down to one question: does the image contain photographic detail or flat areas and sharp edges? Get that right and the rest follows.
- 13How to Edit and Convert PDFs on iPhoneYour iPhone already does more with PDFs than most people realise — and stops dead at the four things people most often need. Knowing the line saves you from installing anything.
- 14How to Edit and Convert PDFs on AndroidAndroid is generous about making PDFs and gives you almost nothing for changing one. The gap is real, but it does not need an app to fill.
- 15How to Get a PDF Under an Exact Size"Make it smaller" and "get it under 200 KB" are not the same problem. The second one has a budget, and the budget divides by the number of pages.
- 16How to Reduce an Image to a Size in KBThere are two levers on image size and most people only pull one. Compression is the famous one. Resizing is the one that does the work.
- 17How to Scan a Document With Your PhoneA photo of a page and a scan of a page are different things. The difference is edge detection, perspective correction and contrast, and both iOS and Android do all three for free.
- 18How to Sign a PDF Without Printing ItThree quite different things get called "signing a PDF", and picking the wrong one either wastes an afternoon or gives you something that will not hold up.
- 19How to Use PDF Tools With No InternetPeople search for offline PDF tools for two very different reasons, and the same setup answers both — but only if you visit once before you need it.
- 20How to Edit and Convert PDFs on iPadAn iPad is much closer to a real PDF workstation than an iPhone is — drag and drop works, the Pencil is a genuine signing tool, and Safari loads desktop sites by default. Four jobs still stop it dead.
- 21How to Sign a PDF on iPadFor once the obvious answer is the right one: Markup, with a Pencil, is genuinely excellent. The part worth knowing is what Markup leaves behind in the file afterwards.
- 22How to Merge PDFs on an iPadThere is a way to do this without installing anything, and it is hidden inside Markup. It is also slow enough that it is only worth using for two files.
- 23How to Convert PDF to Word on iPadThis is the gap in iPadOS that sells the most subscriptions. The conversion itself can run in Safari — but which of the two kinds of PDF you are holding decides whether it will work at all.
- 24How to Generate Sample PDFs for TestingThe PDF you need to test with is almost never the PDF on your desk. That one is confidential, the wrong length, and has none of the properties you are trying to break.
How it works underneath
What the tools are doing to your file — the mechanics of extraction, compression, recognition and encryption.
- 25What's Actually Inside a PDF FileOpen a PDF in a text editor and you will find something closer to a database dump than a document. Almost every odd thing PDFs do follows from that one fact.
- 26How Text Extraction From a PDF WorksThere are no words in a PDF. There are numbered glyphs at coordinates. Everything you get back from a text extractor is an educated reconstruction, which is why it is sometimes uncannily good and sometimes gibberish.
- 27How PDF Compression Actually Works"Compress PDF" runs several unrelated operations behind one button. One of them is lossless and usually already done. One of them is where all the size comes from, and it is not free.
- 28How OCR Turns a Scan Into TextOCR is not one operation. It is a pipeline of five, and almost every disappointing result can be traced to which stage went wrong.
- 29How Merging Two PDFs Actually WorksBoth files have an object numbered 1. That single collision is the whole reason merging is more than concatenation, and it explains everything that does and does not survive the operation.
- 30How JPEG Compression WorksJPEG works by discarding information your eye is bad at noticing. Understanding which information explains why quality 80 looks fine, why text gets halos, and why re-saving is corrosive.
- 31How PDF Password Protection WorksA PDF can carry two passwords, and only one of them encrypts anything. The other is a request that readers are trusted to honour — which is exactly why it can be stripped.
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