The one idea
The letters after the dot tell the computer what kind of thing the file is, and therefore which program should open it and what it can do.
There are two broad families, and almost every format problem comes from confusing them.
| Family | What it stores | What happens when sent |
|---|---|---|
| Editable — docx, xlsx, pptx | Instructions for rebuilding the document | The other computer rebuilds it, possibly differently |
| Fixed — pdf, png, jpg | The finished result | Looks the same everywhere |
Some files can be edited but might look different on another computer. Others always look the same but are harder to change.
Send editable formats when someone must change the content. Send PDF when the layout matters or the document is final.
Editable formats store content plus formatting instructions, resolved at render time against locally available fonts and layout engines. Fixed formats embed the rendered result, including font subsets.
The formats that actually matter
Documents
- docx — Word. Editable. Formatting may shift on another machine.
- pdf — Fixed. Looks identical everywhere. Send this when it is final.
- txt — Plain text, no formatting at all. Opens anywhere, forever.
- md — Markdown. Plain text with simple formatting marks. Increasingly the format AI tools and developers work in.
Data
- xlsx — Excel. Formulas, formatting, multiple sheets.
- csv — Just the values, separated by commas. No formulas, no formatting, no colours. Every data tool on earth can read it. That is the point of it.
Fact A CSV file cannot store formulas, colours or multiple sheets. If you save an
Excel workbook as CSV, you keep one sheet of values and lose everything else.
Excel warns you, in a dialog most people click straight through.
Images
- jpg — Photos. Small file size, slight quality loss. No transparency.
- png — Screenshots, logos, anything with sharp edges or transparency. Larger files.
- svg — Vector. Scales to any size without blurring. For logos and diagrams.
- webp — Modern, smaller than both JPG and PNG. Occasionally rejected by older software.
Archives
- zip — Many files compressed into one. Right-click to extract. Emailing a folder usually requires zipping it first.
You send a spreadsheet with formulas to a colleague who opens it on their phone, edits a number and saves. Formulas may not recalculate the way you expect, and some formatting can be lost. This is a real cause of "the numbers changed and nobody touched them".
A form asks you to upload a photo of a document, maximum 200 KB. Your phone produces a 4 MB file. The fix is not a different phone. It is exporting a smaller JPG, which is what "compress" means in every tool that offers it.
Deciding what to send
1 of 4Ask: does the other person need to change it?
If no, send a PDF. This single question prevents most formatting complaints.
Try this
For each situation, decide what format to send:
- A CV to a recruiter.
- A budget your manager needs to edit.
- A list of 5,000 customer records for the data team.
- A screenshot of an error message.
Your challenge
Level 3 · IndependentTake one document you have. Produce three versions: an editable one, a PDF, and a plain-text version.
Open all three and note what was lost at each step. Then answer: for each version, who would you send it to, and why?
The point is not the conversions. It is noticing exactly what each format throws away.
What people usually get wrong
- Sending an editable file when the layout matters. The most common cause of "it looks broken on my end".
- Saving as CSV and losing the workbook. Excel warns you. Read the warning.
- Emailing a 40 MB attachment. It bounces, or it sits in someone's inbox forever.
- Using JPG for screenshots. Text goes fuzzy. Use PNG.
- Renaming a .docx to .pdf. Changing the extension does not convert the file. It only breaks it.
- Assuming the other person has your software. They may not, and they will not tell you. They will just not open it.
How someone experienced does it
Experienced people send the format that requires the least work from the recipient. That is the whole principle. If someone has to convert, chase or install something to read your file, you have moved your work onto them.
They also name the file so it makes sense in a downloads folder full of other
people's files. Proposal.pdf is invisible.
2026-03-14 Acme proposal v2.pdf is not.
Why a document looks different on another computer
An editable document does not store what you see. It stores instructions: "heading, 18pt, Calibri, bold". When it opens elsewhere, that computer follows those instructions using the resources it has.
If it does not have Calibri, it substitutes another font. The substitute has different letter widths, so lines break in different places, so pages break in different places, so your carefully positioned image is now on page three.
Nothing failed. The document was rebuilt faithfully from instructions that assumed things which were not true on that machine. A PDF avoids this by embedding the fonts and the finished layout — which is also why PDFs are larger and harder to edit.
Prove it
Convert something real and send it to someone. Ask them to confirm it opened correctly on their device.
That confirmation is the skill. The conversion was the easy part.
Keep learning this
Paste this into any AI assistant. It turns the assistant into a tutor that tests you instead of just answering you.
Act as an experienced practitioner who is good at teaching. I have just learned file formats and choosing the right one to send. Assume I am intelligent but relatively new to this — treat me as beginner level. Work through this in order, and wait for my reply at each step: 1. Ask me 5 questions that test whether I actually understood file formats and choosing the right one to send. Do not reveal the answers yet. 2. After I answer, tell me which parts I got right, which I got wrong, and which I only half-understand. Explain only what I misunderstood — do not re-teach what I already know. 3. Give me one practical challenge based on something I could genuinely encounter at work or in daily life. Do not solve it for me. 4. Evaluate my solution the way an experienced person would judge it, including what a professional would have done differently. 5. Tell me what to learn next, and why that comes next. 6. Give me trustworthy sources for deeper study — prefer official documentation, primary research or standards bodies over blogs and videos. Rules for you: no buzzwords. No motivational filler. Say "I'm not certain" when you are not certain, and tell me which parts of your answer I should verify myself. Clearly separate facts from your recommendations and your opinions.
Become independent at this
Use this when you want a path from where you are to actually good, with checkpoints you can test yourself against.
I want to become independently capable at working with file formats and conversions — not permanently dependent on AI, tutorials or step-by-step guides. Design a progression for me with five stages: Beginner, Guided practice, Independent practice, Real-world application, Professional level. For each stage tell me: - what I must know - what I must be able to do without help - the mistakes people make at this stage - one practical challenge - one real project that would prove I reached this stage - one way I can test myself honestly Then tell me the signals that I am ready to move to the next stage, and the signals that I have skipped ahead too early. Keep the theory to the minimum I actually need. Focus on ability I can transfer to situations you and I have not discussed.