There might be cases when some annotation tools, like Highlight, Underline, or Strikeout don't seem to work - the reason might be file-specific. In order to check that, you can do as follows:
Try selecting the text in your file - if it can't be selected, or your file is not searchable, your file is most likely a scan or an image that was saved as a PDF, but not converted properly. Apply Text Recognition and check if it helps.
If the text in your file can be selected properly, but some tools are still not applied, please send us a sample file to rdsupport@readdle.com. If the file's size is too big for sending it over email, you can upload it to any cloud storage service and share a link with us - e.g., you can use the WeTransfer service.
Try selecting the text in your file - if it can't be selected, your file is most likely a scan or an image that was saved as a PDF, but not converted properly. If you have access to PDF Expert for Mac, or any other OCR processor, apply Text Recognition and check if it helps.
If not, here's a workaround we suggest: use Pen Tool to highlight the needed text (change the opacity of the tool to make it more transparent, if needed).
If the text in your file can be selected properly, but some tools are still not applied, please send us a sample file to rdsupport@readdle.com. If the file's size is too big for sending it over email, you can upload it to any cloud storage service and share a link with us - e.g., you can use the WeTransfer service.