Instead of using Start Process, why don’t you use Read PDF or Read PDF with OCR activities, this will read your PDF and extract the content in a string format, then you can use Regex expressions to extract your text required
Read pdf can be used if your pdf has only the text
Read pdf with can be used if your pdf has text as a image and the ocr will help to read the text from that image
First go to Design Tab-> Manage Packages → All Packages tan and search for UiPath.PDF.Activities
Install it
And back in studio search for PDF in activity panel and from there you can pick the activity we need