PDF Editing Issues

Fix Unable to Fill Out PDF Form on iPhone in Acrobat Sign

Maira Mehtab Oct 19, 2026 868 views

Question details

The user is unable to complete or send a PDF form using Acrobat Sign on an iPhone due to an error stating that a custom field is empty or marked as required.

How to Fix 'Unable to Fill Out PDF Form' in Acrobat Sign on iPhone

Product: Adobe Acrobat Sign
Device & OS: iPhone
Scenario: Attempting to fill out and submit a PDF form on a mobile device.
Observed behavior: An error message blocks the document submission process, indicating that a specific field, such as Custom Field 3, requires input before sending.

Before you start

Verify that your iPhone has a stable internet connection and ensure your PDF viewing application is updated to the latest version before troubleshooting form errors.

Solution 1Recommended

Check and Complete Required Custom Fields

Identify missing mandatory fields in the PDF document and either fill them with valid data or change their required status using a desktop PDF editor.

PDF forms created with mandatory interactive fields will block the submission process if left empty. Mobile apps will strictly enforce these document rules.

Review the error message

Check the exact name of the field mentioned in the iPhone error prompt, such as 'Custom Field 3', to identify what is missing.

Fill the missing field

Scroll carefully through the PDF document on your iPhone to locate the required field and enter the necessary information.

Open document on a computer

If the field is not needed but blocks submission, open the same PDF file on a Windows or macOS computer using Adobe Acrobat Pro.

Modify field properties

Right-click the problematic custom field, select 'Properties', and uncheck the 'Required' box. Save the document and try opening it again on your iPhone.

Check and Complete Required Custom Fields

Cross-Device Testing: To isolate the problem, try opening and testing a different PDF form on your iPhone to determine if the issue is specific to a single document.

Fill PDF Forms Easily on Mobile

Use WPS Office to Seamlessly Fill and Sign PDF Forms on iPhone

Skip complicated form errors. WPS Office provides a powerful, mobile-friendly PDF reader and editor that lets you easily fill, sign, and share PDF forms directly from your iPhone without layout issues.

1. Install WPS Office: Download and install the free WPS Office app from the App Store on your iPhone.
2. Open the PDF File: Launch WPS Office and open the PDF form you need to complete.
3. Fill in the Form Fields: Tap on the interactive text fields to securely enter your data, or select the 'Fill & Sign' tool from the bottom menu to add digital signatures.
4. Save and Share: Tap 'Save' once the form is completed, then use the share icon to instantly send the finished document via email or messaging apps.

Smooth interactive PDF form filling and signing directly on mobile devicesFully compatible with standard Adobe PDF and Microsoft Office document formatsLightweight application with a familiar interface that saves storage space on your iPhone

microsoft office alternative - wps office

Frequently Asked Questions

Why does Acrobat Sign say a custom field is required when I try to send?

When the original author created the PDF form, they marked specific interactive fields as mandatory. The application enforces this rule and prevents you from submitting the document until all required fields contain data.

Can I remove the 'required' status of a PDF field directly on my iPhone?

Typically, mobile PDF viewer apps only allow you to fill in fields, not edit their core structure. You will need to open the document in a desktop PDF editor, such as WPS Office Premium or Adobe Acrobat Pro, to change the required status.

What if I cannot locate the missing custom field on my mobile screen?

Mobile screens can sometimes truncate or hide poorly formatted PDF fields. Try zooming out, viewing the document in landscape mode, or opening the file on a desktop computer to easily locate the missing field.

PDF Troubleshooting