Go to the website on your iPad and click the link to open it up.
In the upper right corner you will see the share icon, click on that and scroll down and select Save to Files, choose On My iPad. If you had been using Google Drive before you just need to tap on it to close it and you should see On My iPad.
If the PDFs are emailed to you
Long press on the PDF in the body of the email and you will see a dialog box, choose Save to Files.
You will see another dialog box come up, choose on my iPad. If you had been using Google Drive before you just need to tap on it to close it and you should see On My iPad.
To combine multiple PDFs into one file
Tap on the Files app and choose On My iPad.
Choose Select in the upper right.
Tap on the files you want to combine, you will know they are selected when they have a blue check mark.
Tap More in the lower right corner.
Select Create PDF.
It will create a PDF that has the same name as the first PDF you selected with a different time.
You can change the name of the file by tapping on the name, which will highlight it, just start typing the new name.
To Fill Out Your PDFs
Once you have your PFDs combined, tap the file to open it.
In the top right hand corner you will see the Markup Icon, tap on that and you can use your Apple Pencil to fill in the PDF and sign it.