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The routine in Guide 3 works, but it took six prompts. A skill is that routine written down once, in a form Claude finds and follows whenever a PDF comes into a matter. This guide builds one, with Claude doing most of the typing, and shows how to install, test and keep it.
What a skill is
A skill is a folder. Inside it is a file called SKILL.md, which is a plain-text document with a short header and then instructions, written the way you would brief a junior. The folder may also hold scripts and reference files, but the first one you build needs neither.
The header has two required lines. name is a short lower-case label with hyphens, no spaces, up to 64 characters, and it must match the folder’s name. description says what the skill does and when to use it. Claude reads the descriptions of all the skills it has and loads the one whose description matches the task. If the description is vague, the skill will not be used when it should be, or will be used when it should not.
Below the header come the instructions: the steps, in order; what to produce; what never to do; and, if it helps, an example of a good result. Keep the file under about 500 lines. Detail that would make it longer goes into a separate reference file that the instructions point to.
That is the whole format. It is an open standard, so a skill written for Claude can be read by other tools that adopt the same format.
Why write one
Three reasons.
Consistency. The routine runs the same way on every document, including on the day you are in a hurry. The checks in Guide 3 are exactly the kind of thing that gets skipped under pressure.
Auditability. A skill is a file a colleague can open and read. If the question is ever asked how a document was converted and checked, the answer is in the file, not in a conversation from three months ago.
Time. The third time you type “make an image of every page, run OCR, keep the images, compare the flagged pages, mark what you cannot read”, you realise you will be typing it for the rest of your career.
Step 1 — Write the brief in plain language first
Before anything technical, write down, in your own words, what the routine is. For the document-reading skill it is the sequence from Guide 3:
Look at each page and flag the risky ones. Pull the text out, with the right method for the kind of PDF. Keep a picture of every page. Compare the text of every flagged page with its picture and fix the text from the picture. Mark what cannot be read. Write a note at the top of the result saying which pages were checked and what is still uncertain. Never change the original PDF.
If you can write that paragraph, you can write the skill. The rest is formatting.
Step 2 — Ask Claude to write the skill file
Start a Cowork session with any folder connected (a scratch folder is fine; the skill file will be created there). Copy this prompt.
Write a skill for me as a folder called pdf-intake containing a SKILL.md file. The header must have "name: pdf-intake" and a one-sentence description that says the skill turns any PDF into a checked text file, with OCR for scans, and should be used whenever I ask to read, convert, extract or OCR a PDF. The instructions must follow these steps in order: (1) look at every page and flag pages with rotated text, tables, several columns, stamps, handwriting, watermarks, faint scans or easily confused characters; (2) extract the text, using the text layer for born-digital PDFs and OCR for scans; (3) save an image of every page in a folder next to the output; (4) compare the text of every flagged page against its image and correct the text from the image, recording each correction; (5) mark anything unreadable as [?]; (6) write the result as a text file with one heading per page, and a note at the top listing the pages checked by eye and the [?] count. Rules: never modify, rename or delete the original PDF; never guess at an unreadable word; never report a page as checked unless its image was compared. Keep SKILL.md under 150 lines and write it so that a lawyer can read it.
Claude writes the folder and the file. It will usually add a short script for the page-flagging step; that is fine, and the file will say what the script does.
Step 3 — Read it, and change what you do not like
Open SKILL.md in your text editor and read it as you would read a junior’s note of instructions before signing it. A made-up example of what the top of the file should look like:
---
name: pdf-intake
description: Turn any PDF into a checked text file, with OCR for scans and a comparison of risky pages against the page image. Use whenever asked to read, convert, extract or OCR a PDF.
---
# PDF intake
Rule: no extracted text is trusted until the risky pages have been compared with the page image.
## Step 1 — Assess
For every page, flag: rotated text; tables; several columns; stamps, signatures or handwriting; watermarks; faint scans; identifiers with 0/O, 1/I or 5/S.
## Step 2 — Extract
Born-digital PDF: read the text layer. Scan: OCR each page image. Handwriting: read from the image; use OCR only as a cross-check.
...
Check four things. The description says when to use the skill, in words you would actually type. The steps are in the order you want. The rules that must never be broken are stated as rules, not preferences. And the output is described precisely enough that you would recognise a wrong one.
Change anything you dislike, save, and keep going. Skills improve by being used on real documents and corrected; the useful ones are the ones that have failed and been fixed.
Step 4 — Package it
The folder must be named exactly as the name line says: pdf-intake. Make a zip file of the folder, so that the folder itself is at the top level of the zip, with SKILL.md inside it. A zip that contains the files loose at the top level, without the folder, will not install.
pdf-intake.zip
└── pdf-intake/
├── SKILL.md
└── scripts/ (only if Claude added one)
Step 5 — Install and enable it
Open Customize in the sidebar and choose the Skills tab. Use the + button to add a skill, and upload the zip file. The skill appears in your list with a switch next to it; make sure the switch is on.
Two details worth knowing. Skills you upload are private to your account unless you share them. And Cowork loads the skills that are enabled for your account at the start of each session, so if you add or edit a skill during a session, start a new session before you expect the change to apply.
On a Mac, on the Pro, Max and Team plans, Cowork also offers a way to record yourself doing a task and have Claude propose a skill from the recording. It keeps screenshots of the session, so do not record with client material on screen. For a routine like this one, writing the brief is quicker and gives you a file you have actually read.
Step 6 — Test it
Start a new Cowork session on a folder with a scanned PDF in it, and type the kind of request the description promises to catch:
Convert the scanned PDF in this folder to text.
Claude should announce that it is using the skill, or you should see it in the account of its reasoning. If it does not use the skill, the description is the first thing to change: make it closer to the words you actually used. If it uses the skill but skips a step, the instruction for that step is too soft; turn it into a rule.
Then check the output as Guide 3 says: the page images are there, the flagged pages have been compared, the [?] marks are honest. A skill that reports “all pages checked” when the images were never opened is worse than no skill, and this is the test that catches it.
You can also ask Claude to edit the skill for you in a chat, but read every change before you accept it. The skill is yours, and you are the one who will answer for what it produced.
Step 7 — Keep it as a file you control
Keep every skill as a folder in a place you back up, with a date in a note at the top of the file and a line for each change. When a skill fails on a real document, fix the file the same day. Over a year the collection of skills becomes a record of how you actually work with these tools, which is worth having when a colleague, a client or a court asks.
What a skill will not do
It will not check its own work. A document-reading skill will tell you which pages it compared with their images; it will not tell you that its comparison was careless. It will not stop you using it on the wrong material: run it on a folder of unreadable scans and it will produce tidy text for the pages it could read. Skills make the mechanical parts repeatable. They do not take over the part that is yours.
Check yourself
At the end of this guide you should have a folder called pdf-intake with a SKILL.md you have read and edited; the same folder zipped, uploaded under Customize > Skills, and switched on; one scanned PDF converted by the skill, with page images and an honest note at the top; and a note of the date and version in the file.
The worked example PDF intake shows a fuller version of this skill on a made-up bundle. Guide 5 covers plugins, which are the way skills are packaged and shared with others.
Sources: Anthropic documentation, Creating custom skills; support articles, How to create custom skills, Use skills in Claude, Cowork overview (as at 24 September 2026).