A Two-Arm Study to Evaluate the Pharmacokinetics, Efficacy, and Safety of Subcutaneous Administration of the Fixed-Dose Combination of Pertuzumab and Trastuzumab in Combination With Chemotherapy in Chinese Participants With HER2-Positive Early Breast Cancer

Pertuzumab will be administered as a fixed non-weight-based loading dose of 840-milligrams (mg) IV and then a 420-mg IV maintenance dose Q3W.

Trastuzumab will be administered as an 8-milligram per kilogram of body weight (mg/kg) IV loading dose and then 6 mg/kg IV maintenance dose Q3W.

The pertuzumab and trastuzumab fixed-dose combination for subcutaneous administration (PH FDC SC) will be administered SC at a fixed non-weight-based dose. A loading dose of 1200 mg SC pertuzumab and 600 mg SC trastuzumab is then followed by a maintenance dose of 600 mg SC pertuzumab and 600 mg SC trastuzumab Q3W.

Doxorubicin 60 milligrams per meter squared of body surface area (mg/m\^2) will be administered IV on Day 1 of each cycle of treatment (as part of AC Q3W) for Cycles 1-4.

Cyclophosphamide 600 mg/m\^2 will be administered IV on Day 1 of each cycle of treatment (as part of AC Q3W) for Cycles 1-4.

Docetaxel 75 mg/m\^2 will be administered IV on Day 1 of Cycle 5. At the investigator's discretion the dose may be escalated to 100 mg/m\^2 IV for Cycles 6-8 (Q3W) provided no dose-limiting toxicity occurs.

Participants in both cohorts are scheduled to undergo surgery after 8 cycles of neoadjuvant therapy. Participants may undergo breast-conserving surgery or mastectomy according to routine clinical practice.

If indicated, radiotherapy is given after chemotherapy and surgery, during adjuvant HER2-targeted therapy and hormone therapy (for hormone-receptor positive disease).

For hormone receptor positive breast cancer, tamoxifen or aromatase inhibitors will be allowed as adjuvant hormone therapy for postmenopausal participants and with ovarian suppression or ablation for premenopausal participants in countries where it has been registered for this indication. Its use must be consistent with the registered label. Hormone therapy is given after chemotherapy and surgery during adjuvant HER2-targeted therapy.