How to make a scanned PDF searchable
Text recognition reads the words off each page image, and they are written back into the PDF invisibly, in the place they were read from. The file still looks exactly like a scan, but find-in-page, copy and paste, and screen readers all work on it.
This is the one thing the web tools here do not do. It is in the scanmydocz Android app, which is said plainly rather than implied otherwise.
Why a scan is not searchable to begin with
A scanner produces a picture of a page. To a computer it is pixels: there is no letter 'e' anywhere in the file, only dark shapes. Searching it finds nothing because there is nothing to find. This is also why a scanned PDF weighs so much more than a typed one, and why a screen reader falls silent on it: everything a computer could have used the text for needs the text, and the scan has none.
What a text layer actually is
Every recognised word is drawn onto the page a second time, in a colour with zero opacity, positioned where the ink is. You see the scan; anything that extracts text sees the words. It is an old, completely standard technique, and it is why a searchable PDF looks identical to the scan it came from.
The invisible layer is also why quality varies so much between tools: the picture always looks fine, because the picture was never touched. The differences are all in the part you cannot see, and the only way to judge it is to search for a word you know is on the page, copy a sentence out, and look at what arrives.
What you can actually use today
This site's web tools cannot do it, and scanmydocz's Android app, which can, is in closed testing with no public listing, so it would be no answer to send you there. Here is what exists now, honestly labelled.
On a computer, ocrmypdf is the standout free option: open-source, runs entirely on your own machine, and built for exactly this job, adding a text layer to an existing scanned PDF. It is a command-line tool, which is a real hurdle for some readers, but nothing about your document leaves your computer. Adobe Acrobat does the same job with a friendlier face and a subscription price.
The route most people already have is Google Drive: upload the scan, open it with Google Docs, and the recognised text appears. It works, it is free, and it means handing the document to Google, which is a fine trade for a recipe and a poor one for a contract. Every option in this paragraph and the one above involves that same question, where does the document go, and the answer is the real difference between them.
What recognition gets wrong
Handwriting, mostly. Also poor lighting, unusual layouts, tables, and any script the model was not trained on. Recognition is a machine reading a photograph, and on a bad photograph it reads badly. A good tool tells you how much it managed rather than implying it read everything. Treat any recognised text you are about to rely on, an account number, a date, a name, as something to check against the picture, because the picture is the document and the layer is only a reading of it.
The language limit worth knowing
On-device recognition usually ships one script. scanmydocz's reads Latin, so an English or Urdu-in-Roman page works, and a page in Urdu's own script exports as a perfectly good picture with nothing searchable in it. The app says so rather than letting you discover it in a viewer. If your documents are in Urdu script, Arabic or another non-Latin script, ask any tool you are considering the same question before you pay for it.
The scan itself still has to be good
Recognition cannot read what the camera did not capture. A page shot at an angle, lit from behind you, or photographed on a surface the same colour as the paper produces a picture that is hard for a person to read and harder for a machine. Getting the photograph right is most of the work.
Getting a good scan from a phone photo
Questions people ask next
Can I make a PDF searchable on this website?
No, and pretending otherwise would be the easy thing to do. The web tools produce PDFs that hold page images. Text recognition is in the Android app, which is in closed testing.
Does making it searchable change how the page looks?
No. The recognised words are drawn invisibly over the picture, so the document looks exactly as it did. That is the whole point of the technique.
Will it read Urdu script?
Not today. The recognition ships Latin script, so Urdu written in its own script comes out as a good picture with nothing searchable behind it. Urdu written in Roman letters reads fine.
Can it read handwriting?
Mostly no, and be suspicious of tools that say otherwise. Printed type in good light reads well; neat block capitals read patchily; ordinary joined handwriting defeats the recognition most tools ship. For a handwritten page, the picture is the useful artefact, and a transcription is a job for a person.
Does OCR make the file bigger?
Slightly. The invisible layer is text, and text is cheap: a few kilobytes on a page whose image weighs hundreds. If a tool returns a dramatically larger file, it re-encoded your page images along the way, which is a different change and worth noticing.