If you want to update the file attached to the item then setup the following flow to get this working:

Important: Only the files created using the workflows can be updated.

Step 1: Event

Click on "+Add Step", select "Event" and then "+Add".

Select the target application, event type (the flow initiates as per the event selected e.g. if "When item is created" is selected, the flow will initiate when item is created) and the filter conditions (optional).

Step 2: PDF

Click on "+Add Step" ,select "PDF" and then click on "+Add".

Set the options for creating the PDF file. Set the Page layout using the “Layout” setting, page size using the “Size” setting. Page margins for PDF are set to a default value of 10 mm and they can be modified if needed by using the page margin settings.

Step 3: Update Item

Click on "+Add Step" ,select "Update Item" and then click on "+Add".

Type in the values in the field in case you want to update any field and check the PDF page option from "Attach file/s from step".

Step 4: Workflow Type

Select the workflow type as "Recurring, every time an app item is changed". Everytime an app item is changed attached file will get updated as well.

Step 5: Save> Test> Publish

Save the workflow after all configuration by clicking on the Save button ,Test it and Publish the workflow.

Go to the next article: Creating new item from workflow