The thing that has been sent to you is called a form by everybody involved, and there are three quite different objects that get that name. Which one you have decides whether you are typing, drawing, or reaching for a printer.
One: a real form, with fields
A PDF can carry AcroForm fields — actual interactive widgets with names, types and values, sitting on top of the page. Click one and a cursor appears. These are the ones that work the way everyone expects, and they are the minority.
When they misbehave it is usually one of two things. Either the viewer is a browser preview with partial support, in which case a proper PDF reader will fix it. Or the fields exist but are marked read-only, which happens when the sender filled them in and locked them before sending.
Two: an XFA form
Adobe built a second, incompatible form system on XML and shipped it inside PDFs. If you have ever opened a government or bank document and been told that you need a newer version of a specific reader, you have met one.
XFA is deprecated and was removed from the PDF 2.0 standard, but files produced years ago do not stop existing. Almost nothing outside Adobe's own desktop reader implements it. If a document demands that reader by name, this is why, and there is no clever workaround — treat it as the third kind below, or install the thing it asks for.
Three: a picture of a form
This is most of them. Someone designed a form, printed it or exported it flat, and the result has no fields at all. The boxes are rectangles drawn on the page. The lines are lines. Clicking does nothing because there is nothing to click.
A scan of a printed form is the extreme version — the whole page is one image, so there is not even a rectangle, just pixels arranged to look like one.
The traditional answer is to print it, write on it, and scan it back. That is three devices and twenty minutes to add nine words to a page.
What "fill and sign" actually means
The tools that solve the third case do not fill anything in, because there is nothing to fill. They draw on top of the page. Your text becomes a new text object positioned where you put it; your signature becomes a small transparent image placed at the coordinates you chose.
The result is a normal PDF whose page happens to have more things drawn on it than before. Nothing underneath changes — the original text, the other pages, and any real form fields are untouched. That is why this works on a scan: you are not editing the image, you are putting something in front of it.
It is also why the result looks exactly like the printed-and-scanned version to whoever receives it, without the printer. A signature drawn onto the page is the same artefact you would have produced with a pen and a scanner, and is what almost every request to "sign and return" is asking for.
What flattening is for
If you did have a real form and you have filled it in, the values are stored in the field objects rather than drawn on the page. That has two consequences people discover late.
The first is that a different viewer may render them differently, or not at all. The second is that they are still editable — the recipient can change what you typed, and the field remembers earlier values in some producers.
Flattening draws the current values onto the page and removes the fields. The document then looks the same everywhere, cannot be edited by clicking, and carries no field data. Do it before sending anything you consider final. Do not do it before you are finished, because it is not reversible.
What a drawn signature is worth
It is worth being precise here, because one word covers two very different things.
A drawn signature is a picture of your handwriting on a page. In many jurisdictions that counts as an electronic signature and is accepted for ordinary agreements — a tenancy form, a delivery authorisation, a school letter. It is the direct equivalent of signing a printout, and it carries exactly the same weight, which for most documents is enough.
A digital signature is a cryptographic operation. It binds the document to a certificate issued by an authority, proves the file has not changed since, and can be verified by anyone. It is what a regulator means by a qualified electronic signature.
The second genuinely requires a third party — a certificate authority is a third party by definition — so it cannot be done privately in a browser, and nothing that claims otherwise is doing what it says. If a contract may be disputed, use a service built for it. If a landlord wants the form back by email, draw on it.
Practical notes
Draw the signature on a phone. A finger on glass produces a far better line than a mouse, and the difference is not small — people routinely conclude their handwriting has deteriorated when what has actually happened is that they are drawing with a brick.
Sign the last page unless told otherwise; that is where the line almost always is. If the placement comes out wrong, start again from the original file rather than signing the signed copy, so nothing accumulates.
And keep the unsigned original. A signed PDF is the one you send, not the one you file.
The short version
Click a box. If a cursor appears it is a real form — type, then flatten before sending. If a message names a specific reader, it is XFA and you are not going to win that one. If nothing happens, it is a picture of a form, and drawing on top of it is not a workaround but the correct answer.