Click where an answer should go. If a cursor appears in a box, it is an interactive form and you can type. If nothing happens, it is a flat form and you add text boxes on top.
If it shows a message telling you to open it in Acrobat, it is an XFA form, which almost nothing else supports.
Forms are the most common reason anyone needs to do something to a PDF beyond reading it, and they are also where the format's history is most visible. Three generations of form technology are still in circulation and they have very little in common.
The three kinds
Interactive forms
The standard kind, technically called AcroForms. Real form fields are defined in the document with types, names and positions. Click one and you get a cursor. Tab moves between them. Checkboxes toggle and dropdowns open.
These are well supported almost everywhere: desktop readers, browser viewers, mobile readers and browser-based tools. If your form behaves like this, filling it is straightforward and the only question is saving.
Flat forms
A form in appearance only. Someone designed it, printed it or exported it without interactive fields, or it was scanned from paper. The lines and boxes are drawn on the page like any other graphics and there is nothing to click.
You fill these by adding text on top, positioning a text box over each blank and typing into it. It feels improvised and it produces a perfectly good completed form, which is what matters.
XFA forms
An Adobe technology that puts the real form inside an XML payload, leaving a static PDF page underneath. Government and financial institutions issued a lot of them. Browsers and most readers can only show the placeholder page, which is usually a message asking you to open the file in Acrobat.
- Cursor appears in a boxInteractive form
- Tab moves between fieldsInteractive form
- Nothing happens at allFlat form
- Whole page selects as an imageScanned flat form
- Please wait message appearsXFA form
Filling a flat form
This is the case people find awkward and it is entirely workable with a little care about alignment.
Place a text box over each blank
In the PDFslime editor, add a text box for each blank and position it on the line rather than above or below it. Matching the baseline of nearby printed text is what makes the result look deliberate.
Match the type to the form
A sensible size in a plain typeface, in black. The aim is legibility rather than mimicry, and text that is obviously typed reads as a completed form rather than an altered one.
Flatten before sending
This turns your text boxes into ordinary page content, so nothing can be moved or deleted accidentally by the recipient and the values cannot fail to display in their reader. The PDFslime editor does this when you download. Other editors often save text boxes as comments, and those need their own flatten or print-to-PDF step.
Making the answers stick
Filling a form and finding the fields empty when it is reopened is the classic frustration, and it has two distinct causes.
The first is historical. Older readers could display and fill a form but not save the values unless the document had been specially enabled by Acrobat. Current readers do not have this restriction, and documents produced under that assumption still circulate.
The second is browser viewers. Filling a form in a browser tab and clicking the download button sometimes saves the original file rather than your filled version, because the values live in the viewer rather than in the file. Downloading first, filling in a proper tool, then saving, avoids the question entirely. The fuller diagnosis, with the rarer causes, is in why PDF form fields will not save.
If it is an XFA form
This is the case other software cannot handle, because the content is not in the PDF layer at all. There is nothing for another tool to render, so no amount of software choice helps.
The practical options are Adobe Reader itself, which supports the format, or asking the issuer for an alternative. Many organisations that used XFA have since moved to ordinary interactive forms or web forms, and a request often produces one. Printing the placeholder page and filling it by hand is the fallback, and for many forms it remains an accepted route.
Fill a flat form in your browser
Type over the blanks, add a signature, and download a file with your answers drawn into the page. Nothing is uploaded.
Common questions
Filling and saving reliably.
01How do I know whether my form has real fields?
Click where an answer should go. If a text cursor appears and you can type, the form is interactive. If nothing happens, it is a flat form and you will need to add text on top.
02How do I fill a form with no fields?
Place text boxes over the blanks and type into them, aligning each with the printed line. Flatten the result before sending so your entries become part of the page and cannot be moved or lost.
03Why do my answers disappear when I reopen the file?
Either the document is an older one that expected special rights to allow saving, or you filled it in a browser viewer and downloaded the original rather than your filled copy. Download the file first, fill it in a proper tool, and save.
04What is an XFA form?
An Adobe format that stores the real form in an XML payload with only a placeholder page in the PDF layer. Most readers and all browsers can only show the placeholder. Adobe Reader supports it, or you can ask the issuer for an ordinary PDF.
05Should I flatten a completed form?
Usually yes, before sending. Flattening turns the values into page content so they display identically in every reader and cannot be altered accidentally. Keep the unflattened version if you might need to change an answer.
06Can I add a signature to a form?
Yes. The PDFslime editor lets you draw it, type it or upload an image and place it on the line. A drawn or typed signature is a mark on the page, not a certificate signature, so if the form asks for a digital signature, check with the issuer what they accept.