You will not be able to do text markup(such as highlight,underline,etc.) and edit text in the following situations:
PDF document is opened under protected view mode in Foxit PDF Editor
PDF document is PDF/A compliant file and is opened in PDF/A view mode in Foxit PDF Editor.
PDF document has restrictions and commenting is not allowed.
PDF document is image-based or scanned which doesn’t contain editable text.
1. PDF document is opened under protected view mode in Foxit PDF Editor. Protected view mode is a read-only mode in which most functions are disabled to protect your computer from being harmed by files originated from potentially unsafe locations. The protected view feature is only available in V9.0.1 and its later versions.You could turn off Protected View mode in following two ways:
1）Turn off protected view mode for current PDF file in the bubble pop-up
When you open a PDF file under protected view mode, a bubble message would appear at the upper right corner of the document area as shown below. Click on Enable All Features to turn off protected view mode temporarily for the PDF file opened in current tab.
In some cases, the notification messages in Foxit PDF Editor were set not to show the message which affects the next step under General tab in Preferences window. There would not be any bubble message appearing even when a PDF file is opened under protected mode. Then you should turn off protected view mode via Preferences.
2）Turn off protected view mode via Preferences
To set up your application to never open PDF document in protected view mode, please go to File > Preferences > Security, select Off under Protected View, click on OK to apply the change.
2.The document is PDF/A compliant document and is opened in PDF/A view mode in Foxit PDF Editor. You have to turn off the PDF/A view mode before you could add highlight in PDF file. Please click this article to know how to turn off PDF/A view mode.
4. PDF document is image-based or scanned which doesn’t contain editable text. To check if the document has editable text, navigate to Comment > Text Viewer to see if there is any text. If no text appears, that indicates the document is an image-based document. You can try one of following ways:
1) In Version 11, use Convert > Recognize Text function in Foxit PDF Editor to make the file searchable (for commenting) or editable (for editing).
In version before 11, use OCR (Optical Character Recognition) feature in Foxit PDF Editor to make the text seaerchable or editable and enable commenting or editing.
2) Use Area Highlight tool under Comment menu.
I cannot Highlight and Edit on a standard document