Complete a PDF form on your computer and lock the answers

First, find out what you are dealing with

Thursday evening: a letting agent has emailed rental-application.pdf, four pages, due back by Friday noon with your income, two references and a signature. Before typing anything, spend ten seconds on a test that decides the rest of the evening.

Click the empty line next to "Full name". A blinking cursor or a tinted box means the document has interactive fields. No reaction means the lines are only printed graphics. Try a checkbox too, since some forms mix both: real fields on page one, a flat appendix bolted on later.

PDFGym answers the same question for you. Fill PDF form counts the fillable fields on every page, and a flat file gets a plain message saying there is nothing to fill, rather than a guess.

Four kinds of form and what each one needs

Typing your answers in PDFGym

1. Open Fill PDF form and choose the file, or drag it onto the page. It is read inside this browser tab and is never uploaded.
2. Read the summary above the page: how many fields can be filled, and which ones (digital signature fields, action buttons) stay untouched.
3. Click each outlined box on the rendered page and type. Checkboxes, radio groups and dropdown lists work right where they sit. Use the arrows or the page menu to reach page two and beyond.
4. If a box is awkward to hit, open "Use a field list instead" and complete the same fields as a labelled list.
5. Made a mess? "Clear answers" empties every field, and "Restore original answers" brings back whatever the file contained when you opened it.
6. Press Save filled PDF. The tool reopens its own output and confirms each changed answer is visible, every page survived and the original text is still selectable. If a field lost its value, it stops rather than hand you a broken file.
7. Download the result. Its fields stay editable, so a typo spotted tomorrow is still a quick fix.

One quirk: when a box simply stops accepting characters, the form's author set a maximum length for it. The filler respects that limit, so abbreviate instead of fighting it.

No fields? Two ways to fill a flat page

For a one-off, use Add text in Edit PDF: click where the answer starts, type, then pick Helvetica, Times or Courier and a point size close to the printed labels. A 10 pt answer on a form set in 10 pt looks native; 14 pt looks pasted on. The saved answers stay real, selectable text.

For a layout you fill repeatedly, such as a monthly mileage claim, spend fifteen minutes once in Create PDF form. Drag text, long text, checkbox, date and dropdown fields onto the page, name them, and keep the file as a template. Each month you fill a fresh copy instead of lining up text boxes by hand.

Lock the answers before the file leaves your computer

Interactive fields sit in a layer above the page. Some email previews and older viewers draw only the page underneath, so a form that looked complete on your screen can appear empty on theirs. Flatten PDF removes that risk: it writes each field's appearance into the page content and deletes the field, and the text remains selectable rather than turning into a picture.

• Keep two files: rental-application-editable.pdf stays with you, rental-application-final.pdf goes to the agent.
• In Flatten PDF leave "Form fields and their values" ticked. Tick "Comments, highlights and stamps" too if you marked up any pages.
• A visible signature added with Sign PDF is drawn as page content, so it comes through flattening untouched whichever comes first.
• A certificate-based digital signature has to be the very last change. Filling, flattening or editing afterwards invalidates it, and Flatten PDF asks you to confirm before it touches a signed file.
• Open the final file in a second viewer and read every answer once.

Where form filling usually goes wrong

• The answers lived in the viewer, never in the file. A browser's built-in viewer lets you type, but keeps nothing until you save a copy. Save, close, reopen, then trust it.
• A tick that disappears on the other side. A checkbox can store "on" without a drawn tick that every program understands. PDFGym's filler writes a visible appearance for each answer and checks it, and flattening then makes the tick part of the page.
• Flattening the only copy. It cannot be reversed. When the agent asks you to correct your postcode, you want the editable version, not a blank form.
• Text floating above the lines. On flat forms, answers at the wrong size or height look forged. Zoom to 100%, nudge each box onto its rule, use one font.
• The file refuses to save. A password-protected form has to be unlocked before any tool can write answers into it. If you know the password, Unlock PDF removes it from a copy.
• Merging before flattening. Fields can behave differently once documents are combined. Fill, flatten, then merge with the payslips.

When a browser tool is the wrong choice

Some tax, immigration and insurance forms are built as dynamic XFA documents. Outside Adobe software they often show nothing but a notice telling you to wait or to use a different viewer. Those need Adobe Acrobat Reader on a desktop, and no browser tool will fill them properly. The same goes for forms whose totals calculate as you type: the PDFGym filler stores the values you enter and leaves action buttons alone, so do not count on a sum recalculating.

If the sender offers the same form on a portal, use it: your data goes straight into their system. And when an employer's HR system wants a signature tied to your login, sign there; a file signed elsewhere will not carry the proof they need.