The difficulties could be with techniques or with the PhD itself eg lack of access to equipment, experiments not working, balancing workloads, difficult colleagues etc. You want to get across that you are resilient and that you have strategies for overcoming these issues.
Without knowing what the document is, it's a bit difficult to say... I assume it's a paper? I guess you would want to look at the techniques, think if it's relevant to the PhD topic, see if it has any obvious shortcomings, think about the authors, the IF of the paper, novelty etc.